Sendmux Email Inbox API + Sending

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Read Message Content

mailbox_list_content

Read-onlyIdempotent

Retrieve structured email content for a known message ID, including headers, body parts, and attachment metadata. Use when email details like participants or quoted text affect the answer.

Instructions

Use this to read structured content for a known message_id. Prefer it when headers, participants, or body parts affect the answer.

Input Schema

TableJSON Schema

Name	Required	Description
`part`	No	Body part to return. `auto` prefers text when available.
`mailbox_id`	No	Mailbox public ID to target when the credential grants access to more than one mailbox. Omit when the credential is scoped to exactly one mailbox.
`message_id`	Yes	Message ID.
`include_html`	No	When true, include HTML content when available.
`strip_quotes`	No	When true, remove quoted reply text.
`If-None-Match`	No	Weak ETag from a previous response. Returns 304 when unchanged.
`include_links`	No	When true, include links extracted from the body.
`max_body_chars`	No	Maximum body characters to return before truncation.
`include_headers`	No	Header detail to include in the response.
`strip_signature`	No	When true, remove detected email signatures.
`include_attachments`	No	Attachment detail to include. Contents are not returned.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv1.0.14Sep 23, 2026

changedOutput schema / properties / result / allOf

Previous value: -[
-  {
-    "properties": {
-      "meta": {
-        "additionalProperties": {},
-        "properties": {},
-        "type": "object"
-      },
-      "ok": {
-        "enum": [
-          true
-        ],
-        "type": "boolean"
-      }
-    },
-    "required": [
-      "ok",
-      "meta"
-    ],
-    "type": "object"
-  },
-  {
-    "properties": {
-      "data": {
-        "allOf": [
-          {
-            "additionalProperties": false,
-            "properties": {
-              "attachments": {
-                "description": "Attachment metadata only. Attachment contents are not parsed by this endpoint.",
-                "items": {
-                  "additionalProperties": false,
-                  "properties": {
-                    "content_id": {
-                      "description": "Content ID for inline attachments, when present.",
-                      "type": [
-                        "string",
-                        "null"
-                      ]
-                    },
-                    "content_type": {
-                      "example": "application/pdf",
-                      "type": "string"
-                    },
-                    "disposition": {
-                      "example": "attachment",
-                      "type": [
-                        "string",
-                        "null"
-                      ]
-                    },
-                    "download_url": {
-                      "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
-                      "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
-                      "type": "string"
-                    },
-                    "filename": {
-                      "example": "invoice.pdf",
-                      "type": [
-                        "string",
-                        "null"
-                      ]
-                    },
-                    "id": {
-                      "description": "Attachment blob ID.",
-                      "type": "string"
-                    },
-                    "size_bytes": {
-                      "example": 12345,
-                      "type": [
-                        "integer",
-                        "null"
-                      ]
-                    }
-                  },
-                  "required": [
-                    "id",
-                    "filename",
-                    "content_type",
-                    "size_bytes",
-                    "disposition",
-                    "content_id"
-                  ],
-                  "type": "object"
-                },
-                "type": "array"
-              },
-              "body": {
-                "additionalProperties": false,
-                "properties": {
-                  "extracted_links": {
-                    "items": {
-                      "type": "string"
-                    },
-                    "type": "array"
-                  },
-                  "format": {
-                    "enum": [
-                      "text",
-                      "html"
-                    ],
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "html": {
-                    "description": "HTML body when requested. Returned as a JSON string and not as rendered content.",
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "is_truncated": {
-                    "type": "boolean"
-                  },
-                  "quotes_stripped": {
-                    "type": "boolean"
-                  },
-                  "signature_stripped": {
-                    "type": "boolean"
-                  },
-                  "text": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "truncated_at_chars": {
-                    "type": [
-                      "integer",
-                      "null"
-                    ]
-                  }
-                },
-                "required": [
-                  "format",
-                  "text",
-                  "html",
-                  "is_truncated",
-                  "truncated_at_chars",
-                  "signature_stripped",
-                  "quotes_stripped",
-                  "extracted_links"
-                ],
-                "type": "object"
-              },
-              "dates": {
-                "additionalProperties": false,
-                "properties": {
-                  "received_at": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "sent_at": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  }
-                },
-                "required": [
-                  "received_at",
-                  "sent_at"
-                ],
-                "type": "object"
-              },
-              "headers": {
-                "additionalProperties": false,
-                "properties": {
-                  "full": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "name": {
-                          "type": "string"
-                        },
-                        "value": {
-                          "type": "string"
-                        }
-                      },
-                      "required": [
-                        "name",
-                        "value"
-                      ],
-                      "type": "object"
-                    },
-                    "type": [
-                      "array",
-                      "null"
-                    ]
-                  },
-                  "mode": {
-                    "enum": [
-                      "none",
-                      "selected",
-                      "full"
-                    ],
-                    "type": "string"
-                  },
-                  "selected": {
-                    "properties": {
-                      "in_reply_to": {
-                        "type": [
-                          "string",
-                          "null"
-                        ]
-                      },
-                      "message_id": {
-                        "items": {
-                          "type": "string"
-                        },
-                        "type": "array"
-                      },
-                      "references": {
-                        "items": {
-                          "type": "string"
-                        },
-                        "type": "array"
-                      },
-                      "reply_to": {
-                        "items": {
-                          "additionalProperties": false,
-                          "properties": {
-                            "email": {
-                              "example": "agent@example.com",
-                              "format": "email",
-                              "type": "string"
-                            },
-                            "name": {
-                              "example": "Support",
-                              "type": [
-                                "string",
-                                "null"
-                              ]
-                            }
-                          },
-                          "required": [
-                            "email",
-                            "name"
-                          ],
-                          "type": "object"
-                        },
-                        "type": "array"
-                      }
-                    },
-                    "required": [
-                      "message_id",
-                      "in_reply_to",
-                      "references",
-                      "reply_to"
-                    ],
-                    "type": [
-                      "object",
-                      "null"
-                    ]
-                  }
-                },
-                "required": [
-                  "mode",
-                  "selected",
-                  "full"
-                ],
-                "type": "object"
-              },
-              "id": {
-                "description": "Message ID",
-                "type": "string"
-              },
-              "participants": {
-                "additionalProperties": false,
-                "properties": {
-                  "bcc": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "cc": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "from": {
-                    "allOf": [
-                      {
-                        "additionalProperties": false,
-                        "properties": {
-                          "email": {
-                            "example": "agent@example.com",
-                            "format": "email",
-                            "type": "string"
-                          },
-                          "name": {
-                            "example": "Support",
-                            "type": [
-                              "string",
-                              "null"
-                            ]
-                          }
-                        },
-                        "required": [
-                          "email",
-                          "name"
-                        ],
-                        "type": "object"
-                      },
-                      {
-                        "type": [
-                          "object",
-                          "null"
-                        ]
-                      }
-                    ]
-                  },
-                  "reply_to": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  },
-                  "to": {
-                    "items": {
-                      "additionalProperties": false,
-                      "properties": {
-                        "email": {
-                          "example": "agent@example.com",
-                          "format": "email",
-                          "type": "string"
-                        },
-                        "name": {
-                          "example": "Support",
-                          "type": [
-                            "string",
-                            "null"
-                          ]
-                        }
-                      },
-                      "required": [
-                        "email",
-                        "name"
-                      ],
-                      "type": "object"
-                    },
-                    "type": "array"
-                  }
-                },
-                "required": [
-                  "from",
-                  "to",
-                  "cc",
-                  "bcc",
-                  "reply_to"
-                ],
-                "type": "object"
-              },
-              "states": {
-                "additionalProperties": false,
-                "properties": {
-                  "email_state": {
-                    "type": [
-                      "string",
-                      "null"
-                    ]
-                  },
-                  "option_hash": {
-                    "type": "string"
-                  }
-                },
-                "required": [
-                  "email_state",
-                  "option_hash"
-                ],
-                "type": "object"
-              },
-              "subject": {
-                "type": [
-                  "string",
-                  "null"
-                ]
-              },
-              "thread_id": {
-                "type": [
-                  "string",
-                  "null"
-                ]
-              }
-            },
-            "required": [
-              "id",
-              "thread_id",
-              "subject",
-              "participants",
-              "dates",
-              "body",
-              "headers",
-              "attachments",
-              "states"
-            ],
-            "type": [
-              "object",
-              "null"
-            ]
-          },
-          {
-            "type": "object"
-          }
-        ]
-      },
-      "meta": {
-        "additionalProperties": false,
-        "properties": {
-          "request_id": {
-            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
-            "type": "string"
-          }
-        },
-        "required": [
-          "request_id"
-        ],
-        "type": "object"
-      }
-    },
-    "required": [
-      "data"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "properties": {
+      "meta": {
+        "additionalProperties": {},
+        "properties": {},
+        "type": "object"
+      },
+      "ok": {
+        "enum": [
+          true
+        ],
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "ok",
+      "meta"
+    ],
+    "type": "object"
+  },
+  {
+    "properties": {
+      "data": {
+        "additionalProperties": false,
+        "properties": {
+          "attachments": {
+            "description": "Attachment metadata only. Attachment contents are not parsed by this endpoint.",
+            "items": {
+              "additionalProperties": false,
+              "properties": {
+                "content_id": {
+                  "description": "Content ID for inline attachments, when present.",
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "content_type": {
+                  "example": "application/pdf",
+                  "type": "string"
+                },
+                "disposition": {
+                  "example": "attachment",
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "download_url": {
+                  "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
+                  "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
+                  "type": "string"
+                },
+                "filename": {
+                  "example": "invoice.pdf",
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "id": {
+                  "description": "Attachment blob ID.",
+                  "type": "string"
+                },
+                "size_bytes": {
+                  "example": 12345,
+                  "type": [
+                    "integer",
+                    "null"
+                  ]
+                }
+              },
+              "required": [
+                "id",
+                "filename",
+                "content_type",
+                "size_bytes",
+                "disposition",
+                "content_id"
+              ],
+              "type": "object"
+            },
+            "type": "array"
+          },
+          "body": {
+            "additionalProperties": false,
+            "properties": {
+              "extracted_links": {
+                "items": {
+                  "type": "string"
+                },
+                "type": "array"
+              },
+              "format": {
+                "enum": [
+                  "text",
+                  "html"
+                ],
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "html": {
+                "description": "HTML body when requested. Returned as a JSON string and not as rendered content.",
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "is_truncated": {
+                "type": "boolean"
+              },
+              "quotes_stripped": {
+                "type": "boolean"
+              },
+              "signature_stripped": {
+                "type": "boolean"
+              },
+              "text": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "truncated_at_chars": {
+                "type": [
+                  "integer",
+                  "null"
+                ]
+              }
+            },
+            "required": [
+              "format",
+              "text",
+              "html",
+              "is_truncated",
+              "truncated_at_chars",
+              "signature_stripped",
+              "quotes_stripped",
+              "extracted_links"
+            ],
+            "type": "object"
+          },
+          "dates": {
+            "additionalProperties": false,
+            "properties": {
+              "received_at": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "sent_at": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              }
+            },
+            "required": [
+              "received_at",
+              "sent_at"
+            ],
+            "type": "object"
+          },
+          "headers": {
+            "additionalProperties": false,
+            "properties": {
+              "full": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "name": {
+                      "type": "string"
+                    },
+                    "value": {
+                      "type": "string"
+                    }
+                  },
+                  "required": [
+                    "name",
+                    "value"
+                  ],
+                  "type": "object"
+                },
+                "type": [
+                  "array",
+                  "null"
+                ]
+              },
+              "mode": {
+                "enum": [
+                  "none",
+                  "selected",
+                  "full"
+                ],
+                "type": "string"
+              },
+              "selected": {
+                "properties": {
+                  "in_reply_to": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "message_id": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "references": {
+                    "items": {
+                      "type": "string"
+                    },
+                    "type": "array"
+                  },
+                  "reply_to": {
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "email": {
+                          "example": "agent@example.com",
+                          "format": "email",
+                          "type": "string"
+                        },
+                        "name": {
+                          "example": "Support",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "email",
+                        "name"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  }
+                },
+                "required": [
+                  "message_id",
+                  "in_reply_to",
+                  "references",
+                  "reply_to"
+                ],
+                "type": [
+                  "object",
+                  "null"
+                ]
+              }
+            },
+            "required": [
+              "mode",
+              "selected",
+              "full"
+            ],
+            "type": "object"
+          },
+          "id": {
+            "description": "Message ID",
+            "type": "string"
+          },
+          "participants": {
+            "additionalProperties": false,
+            "properties": {
+              "bcc": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "email": {
+                      "example": "agent@example.com",
+                      "format": "email",
+                      "type": "string"
+                    },
+                    "name": {
+                      "example": "Support",
+                      "type": [
+                        "string",
+                        "null"
+                      ]
+                    }
+                  },
+                  "required": [
+                    "email",
+                    "name"
+                  ],
+                  "type": "object"
+                },
+                "type": "array"
+              },
+              "cc": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "email": {
+                      "example": "agent@example.com",
+                      "format": "email",
+                      "type": "string"
+                    },
+                    "name": {
+                      "example": "Support",
+                      "type": [
+                        "string",
+                        "null"
+                      ]
+                    }
+                  },
+                  "required": [
+                    "email",
+                    "name"
+                  ],
+                  "type": "object"
+                },
+                "type": "array"
+              },
+              "from": {
+                "anyOf": [
+                  {
+                    "additionalProperties": false,
+                    "properties": {
+                      "email": {
+                        "example": "agent@example.com",
+                        "format": "email",
+                        "type": "string"
+                      },
+                      "name": {
+                        "example": "Support",
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      }
+                    },
+                    "required": [
+                      "email",
+                      "name"
+                    ],
+                    "type": "object"
+                  },
+                  {
+                    "type": "null"
+                  }
+                ]
+              },
+              "reply_to": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "email": {
+                      "example": "agent@example.com",
+                      "format": "email",
+                      "type": "string"
+                    },
+                    "name": {
+                      "example": "Support",
+                      "type": [
+                        "string",
+                        "null"
+                      ]
+                    }
+                  },
+                  "required": [
+                    "email",
+                    "name"
+                  ],
+                  "type": "object"
+                },
+                "type": "array"
+              },
+              "to": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "email": {
+                      "example": "agent@example.com",
+                      "format": "email",
+                      "type": "string"
+                    },
+                    "name": {
+                      "example": "Support",
+                      "type": [
+                        "string",
+                        "null"
+                      ]
+                    }
+                  },
+                  "required": [
+                    "email",
+                    "name"
+                  ],
+                  "type": "object"
+                },
+                "type": "array"
+              }
+            },
+            "required": [
+              "from",
+              "to",
+              "cc",
+              "bcc",
+              "reply_to"
+            ],
+            "type": "object"
+          },
+          "states": {
+            "additionalProperties": false,
+            "properties": {
+              "email_state": {
+                "type": [
+                  "string",
+                  "null"
+                ]
+              },
+              "option_hash": {
+                "type": "string"
+              }
+            },
+            "required": [
+              "email_state",
+              "option_hash"
+            ],
+            "type": "object"
+          },
+          "subject": {
+            "type": [
+              "string",
+              "null"
+            ]
+          },
+          "thread_id": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "required": [
+          "id",
+          "thread_id",
+          "subject",
+          "participants",
+          "dates",
+          "body",
+          "headers",
+          "attachments",
+          "states"
+        ],
+        "type": "object"
+      },
+      "meta": {
+        "additionalProperties": false,
+        "properties": {
+          "request_id": {
+            "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+            "type": "string"
+          }
+        },
+        "required": [
+          "request_id"
+        ],
+        "type": "object"
+      }
+    },
+    "required": [
+      "data"
+    ],
+    "type": "object"
+  }
+]

Changed4 schema fields changedv1.0.8Sep 15, 2026

addedOutput schema / $schema

Added value: +"https://json-schema.org/draft/2020-12/schema"

removedOutput schema / additionalProperties
```
Removed value: -true
```

addedOutput schema / properties

Added value: +{
+  "result": {
+    "allOf": [
+      {
+        "properties": {
+          "meta": {
+            "additionalProperties": {},
+            "properties": {},
+            "type": "object"
+          },
+          "ok": {
+            "enum": [
+              true
+            ],
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "ok",
+          "meta"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "data": {
+            "allOf": [
+              {
+                "additionalProperties": false,
+                "properties": {
+                  "attachments": {
+                    "description": "Attachment metadata only. Attachment contents are not parsed by this endpoint.",
+                    "items": {
+                      "additionalProperties": false,
+                      "properties": {
+                        "content_id": {
+                          "description": "Content ID for inline attachments, when present.",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "content_type": {
+                          "example": "application/pdf",
+                          "type": "string"
+                        },
+                        "disposition": {
+                          "example": "attachment",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "download_url": {
+                          "description": "Short-lived URL for this exact attachment. Fetch it promptly; if it expires, call the message or attachment metadata endpoint again to receive a fresh URL.",
+                          "example": "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachment-downloads/attref_abc123?download_token=...",
+                          "type": "string"
+                        },
+                        "filename": {
+                          "example": "invoice.pdf",
+                          "type": [
+                            "string",
+                            "null"
+                          ]
+                        },
+                        "id": {
+                          "description": "Attachment blob ID.",
+                          "type": "string"
+                        },
+                        "size_bytes": {
+                          "example": 12345,
+                          "type": [
+                            "integer",
+                            "null"
+                          ]
+                        }
+                      },
+                      "required": [
+                        "id",
+                        "filename",
+                        "content_type",
+                        "size_bytes",
+                        "disposition",
+                        "content_id"
+                      ],
+                      "type": "object"
+                    },
+                    "type": "array"
+                  },
+                  "body": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "extracted_links": {
+                        "items": {
+                          "type": "string"
+                        },
+                        "type": "array"
+                      },
+                      "format": {
+                        "enum": [
+                          "text",
+                          "html"
+                        ],
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "html": {
+                        "description": "HTML body when requested. Returned as a JSON string and not as rendered content.",
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "is_truncated": {
+                        "type": "boolean"
+                      },
+                      "quotes_stripped": {
+                        "type": "boolean"
+                      },
+                      "signature_stripped": {
+                        "type": "boolean"
+                      },
+                      "text": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "truncated_at_chars": {
+                        "type": [
+                          "integer",
+                          "null"
+                        ]
+                      }
+                    },
+                    "required": [
+                      "format",
+                      "text",
+                      "html",
+                      "is_truncated",
+                      "truncated_at_chars",
+                      "signature_stripped",
+                      "quotes_stripped",
+                      "extracted_links"
+                    ],
+                    "type": "object"
+                  },
+                  "dates": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "received_at": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "sent_at": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      }
+                    },
+                    "required": [
+                      "received_at",
+                      "sent_at"
+                    ],
+                    "type": "object"
+                  },
+                  "headers": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "full": {
+                        "items": {
+                          "additionalProperties": false,
+                          "properties": {
+                            "name": {
+                              "type": "string"
+                            },
+                            "value": {
+                              "type": "string"
+                            }
+                          },
+                          "required": [
+                            "name",
+                            "value"
+                          ],
+                          "type": "object"
+                        },
+                        "type": [
+                          "array",
+                          "null"
+                        ]
+                      },
+                      "mode": {
+                        "enum": [
+                          "none",
+                          "selected",
+                          "full"
+                        ],
+                        "type": "string"
+                      },
+                      "selected": {
+                        "properties": {
+                          "in_reply_to": {
+                            "type": [
+                              "string",
+                              "null"
+                            ]
+                          },
+                          "message_id": {
+                            "items": {
+                              "type": "string"
+                            },
+                            "type": "array"
+                          },
+                          "references": {
+                            "items": {
+                              "type": "string"
+                            },
+                            "type": "array"
+                          },
+                          "reply_to": {
+                            "items": {
+                              "additionalProperties": false,
+                              "properties": {
+                                "email": {
+                                  "example": "agent@example.com",
+                                  "format": "email",
+                                  "type": "string"
+                                },
+                                "name": {
+                                  "example": "Support",
+                                  "type": [
+                                    "string",
+                                    "null"
+                                  ]
+                                }
+                              },
+                              "required": [
+                                "email",
+                                "name"
+                              ],
+                              "type": "object"
+                            },
+                            "type": "array"
+                          }
+                        },
+                        "required": [
+                          "message_id",
+                          "in_reply_to",
+                          "references",
+                          "reply_to"
+                        ],
+                        "type": [
+                          "object",
+                          "null"
+                        ]
+                      }
+                    },
+                    "required": [
+                      "mode",
+                      "selected",
+                      "full"
+                    ],
+                    "type": "object"
+                  },
+                  "id": {
+                    "description": "Message ID",
+                    "type": "string"
+                  },
+                  "participants": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "bcc": {
+                        "items": {
+                          "additionalProperties": false,
+                          "properties": {
+                            "email": {
+                              "example": "agent@example.com",
+                              "format": "email",
+                              "type": "string"
+                            },
+                            "name": {
+                              "example": "Support",
+                              "type": [
+                                "string",
+                                "null"
+                              ]
+                            }
+                          },
+                          "required": [
+                            "email",
+                            "name"
+                          ],
+                          "type": "object"
+                        },
+                        "type": "array"
+                      },
+                      "cc": {
+                        "items": {
+                          "additionalProperties": false,
+                          "properties": {
+                            "email": {
+                              "example": "agent@example.com",
+                              "format": "email",
+                              "type": "string"
+                            },
+                            "name": {
+                              "example": "Support",
+                              "type": [
+                                "string",
+                                "null"
+                              ]
+                            }
+                          },
+                          "required": [
+                            "email",
+                            "name"
+                          ],
+                          "type": "object"
+                        },
+                        "type": "array"
+                      },
+                      "from": {
+                        "allOf": [
+                          {
+                            "additionalProperties": false,
+                            "properties": {
+                              "email": {
+                                "example": "agent@example.com",
+                                "format": "email",
+                                "type": "string"
+                              },
+                              "name": {
+                                "example": "Support",
+                                "type": [
+                                  "string",
+                                  "null"
+                                ]
+                              }
+                            },
+                            "required": [
+                              "email",
+                              "name"
+                            ],
+                            "type": "object"
+                          },
+                          {
+                            "type": [
+                              "object",
+                              "null"
+                            ]
+                          }
+                        ]
+                      },
+                      "reply_to": {
+                        "items": {
+                          "additionalProperties": false,
+                          "properties": {
+                            "email": {
+                              "example": "agent@example.com",
+                              "format": "email",
+                              "type": "string"
+                            },
+                            "name": {
+                              "example": "Support",
+                              "type": [
+                                "string",
+                                "null"
+                              ]
+                            }
+                          },
+                          "required": [
+                            "email",
+                            "name"
+                          ],
+                          "type": "object"
+                        },
+                        "type": "array"
+                      },
+                      "to": {
+                        "items": {
+                          "additionalProperties": false,
+                          "properties": {
+                            "email": {
+                              "example": "agent@example.com",
+                              "format": "email",
+                              "type": "string"
+                            },
+                            "name": {
+                              "example": "Support",
+                              "type": [
+                                "string",
+                                "null"
+                              ]
+                            }
+                          },
+                          "required": [
+                            "email",
+                            "name"
+                          ],
+                          "type": "object"
+                        },
+                        "type": "array"
+                      }
+                    },
+                    "required": [
+                      "from",
+                      "to",
+                      "cc",
+                      "bcc",
+                      "reply_to"
+                    ],
+                    "type": "object"
+                  },
+                  "states": {
+                    "additionalProperties": false,
+                    "properties": {
+                      "email_state": {
+                        "type": [
+                          "string",
+                          "null"
+                        ]
+                      },
+                      "option_hash": {
+                        "type": "string"
+                      }
+                    },
+                    "required": [
+                      "email_state",
+                      "option_hash"
+                    ],
+                    "type": "object"
+                  },
+                  "subject": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  },
+                  "thread_id": {
+                    "type": [
+                      "string",
+                      "null"
+                    ]
+                  }
+                },
+                "required": [
+                  "id",
+                  "thread_id",
+                  "subject",
+                  "participants",
+                  "dates",
+                  "body",
+                  "headers",
+                  "attachments",
+                  "states"
+                ],
+                "type": [
+                  "object",
+                  "null"
+                ]
+              },
+              {
+                "type": "object"
+              }
+            ]
+          },
+          "meta": {
+            "additionalProperties": false,
+            "properties": {
+              "request_id": {
+                "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "MailboxMessageContentResponse"
+  }
+}

addedOutput schema / required
```
Added value: +[
+  "result"
+]
```

First observedv1.0.0Jul 13, 2026

TDQS

A4/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is well covered. The description adds that the content is structured and relevant to headers/participants/body parts, but it does not disclose additional behavioral traits such as response truncation, ETag behavior, or mailbox scoping beyond what the schema already lists.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two sentences, front-loaded with the operation and target resource, followed by a concise selection criterion. There is no filler or repetition of schema content.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the output schema and full parameter descriptions, the tool does not need to restate return values or parameter details. The description provides the key usage discriminator, though for an 11-parameter tool it relies heavily on structured fields.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline is 3. The description adds a general semantic cue that headers/participants/body parts matter, but it does not clarify interactions among the many parameters; the schema carries the semantic weight.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb ('read') and resource ('structured content') and constrains the operation to 'a known message_id'. The second sentence, 'Prefer it when headers, participants, or body parts affect the answer,' clearly distinguishes this from sibling search/list operations.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The phrase 'Prefer it when headers, participants, or body parts affect the answer' gives an explicit, actionable condition for when to use this tool. It does not name alternative tools or provide when-not-to-use exclusions, so it stops short of a full 5.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.