Sendmux Email Inbox API + Sending

Official

by Sendmux

Overview Schema Source Related Servers Discussions Admin

Hybrid

List Thread Messages

mailbox_list_thread_messages

Read-onlyIdempotent

List messages inside a known email thread by thread ID, with pagination and sorting options.

Instructions

Use this to list messages inside a known thread. Use message body tools only for messages that need full content.

Input Schema

TableJSON Schema

Name	Required	Description
`sort`	No	Message sort direction within the thread.
`limit`	No	Maximum results (default 50, max 100).
`cursor`	No	Pagination cursor — the `next_cursor` from the previous response.
`thread_id`	Yes	Thread ID.
`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.

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": {
-        "items": {
-          "additionalProperties": false,
-          "properties": {
-            "attachments": {
-              "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
-              "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"
-            },
-            "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"
-            },
-            "flags": {
-              "additionalProperties": false,
-              "properties": {
-                "answered": {
-                  "type": "boolean"
-                },
-                "draft": {
-                  "type": "boolean"
-                },
-                "flagged": {
-                  "type": "boolean"
-                },
-                "seen": {
-                  "type": "boolean"
-                }
-              },
-              "required": [
-                "seen",
-                "flagged",
-                "draft",
-                "answered"
-              ],
-              "type": "object"
-            },
-            "folder_ids": {
-              "items": {
-                "type": "string"
-              },
-              "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"
-                  ]
-                }
-              ]
-            },
-            "has_attachments": {
-              "type": "boolean"
-            },
-            "id": {
-              "description": "Message ID",
-              "type": "string"
-            },
-            "keywords": {
-              "description": "Active message keywords, including system flags and custom labels.",
-              "example": [
-                "$seen",
-                "$flagged",
-                "agent_triaged"
-              ],
-              "items": {
-                "type": "string"
-              },
-              "type": "array"
-            },
-            "preview": {
-              "type": [
-                "string",
-                "null"
-              ]
-            },
-            "received_at": {
-              "type": [
-                "string",
-                "null"
-              ]
-            },
-            "sent_at": {
-              "type": [
-                "string",
-                "null"
-              ]
-            },
-            "size_bytes": {
-              "type": [
-                "integer",
-                "null"
-              ]
-            },
-            "subject": {
-              "type": [
-                "string",
-                "null"
-              ]
-            },
-            "thread_id": {
-              "type": [
-                "string",
-                "null"
-              ]
-            },
-            "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": [
-            "id",
-            "thread_id",
-            "folder_ids",
-            "subject",
-            "preview",
-            "from",
-            "to",
-            "cc",
-            "bcc",
-            "received_at",
-            "sent_at",
-            "size_bytes",
-            "has_attachments",
-            "keywords",
-            "flags"
-          ],
-          "type": "object"
-        },
-        "type": "array"
-      },
-      "meta": {
-        "additionalProperties": false,
-        "properties": {
-          "request_id": {
-            "type": "string"
-          },
-          "sync_state": {
-            "type": "string"
-          },
-          "thread_id": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "request_id",
-          "thread_id"
-        ],
-        "type": "object"
-      },
-      "pagination": {
-        "additionalProperties": false,
-        "properties": {
-          "has_more": {
-            "type": "boolean"
-          },
-          "next_cursor": {
-            "type": "string"
-          }
-        },
-        "required": [
-          "has_more"
-        ],
-        "type": "object"
-      }
-    },
-    "required": [
-      "data",
-      "pagination"
-    ],
-    "type": "object"
-  }
-]New value: +[
+  {
+    "properties": {
+      "meta": {
+        "additionalProperties": {},
+        "properties": {},
+        "type": "object"
+      },
+      "ok": {
+        "enum": [
+          true
+        ],
+        "type": "boolean"
+      }
+    },
+    "required": [
+      "ok",
+      "meta"
+    ],
+    "type": "object"
+  },
+  {
+    "properties": {
+      "data": {
+        "items": {
+          "additionalProperties": false,
+          "properties": {
+            "attachments": {
+              "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
+              "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"
+            },
+            "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"
+            },
+            "flags": {
+              "additionalProperties": false,
+              "properties": {
+                "answered": {
+                  "type": "boolean"
+                },
+                "draft": {
+                  "type": "boolean"
+                },
+                "flagged": {
+                  "type": "boolean"
+                },
+                "seen": {
+                  "type": "boolean"
+                }
+              },
+              "required": [
+                "seen",
+                "flagged",
+                "draft",
+                "answered"
+              ],
+              "type": "object"
+            },
+            "folder_ids": {
+              "items": {
+                "type": "string"
+              },
+              "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"
+                }
+              ]
+            },
+            "has_attachments": {
+              "type": "boolean"
+            },
+            "id": {
+              "description": "Message ID",
+              "type": "string"
+            },
+            "keywords": {
+              "description": "Active message keywords, including system flags and custom labels.",
+              "example": [
+                "$seen",
+                "$flagged",
+                "agent_triaged"
+              ],
+              "items": {
+                "type": "string"
+              },
+              "type": "array"
+            },
+            "preview": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "received_at": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "sent_at": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "size_bytes": {
+              "type": [
+                "integer",
+                "null"
+              ]
+            },
+            "subject": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "thread_id": {
+              "type": [
+                "string",
+                "null"
+              ]
+            },
+            "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": [
+            "id",
+            "thread_id",
+            "folder_ids",
+            "subject",
+            "preview",
+            "from",
+            "to",
+            "cc",
+            "bcc",
+            "received_at",
+            "sent_at",
+            "size_bytes",
+            "has_attachments",
+            "keywords",
+            "flags"
+          ],
+          "type": "object"
+        },
+        "type": "array"
+      },
+      "meta": {
+        "additionalProperties": false,
+        "properties": {
+          "request_id": {
+            "type": "string"
+          },
+          "sync_state": {
+            "type": "string"
+          },
+          "thread_id": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "request_id",
+          "thread_id"
+        ],
+        "type": "object"
+      },
+      "pagination": {
+        "additionalProperties": false,
+        "properties": {
+          "has_more": {
+            "type": "boolean"
+          },
+          "next_cursor": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "has_more"
+        ],
+        "type": "object"
+      }
+    },
+    "required": [
+      "data",
+      "pagination"
+    ],
+    "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": {
+            "items": {
+              "additionalProperties": false,
+              "properties": {
+                "attachments": {
+                  "description": "Attachment metadata for this message. Each item includes a short-lived `download_url`; if it expires, fetch message metadata again.",
+                  "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"
+                },
+                "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"
+                },
+                "flags": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "answered": {
+                      "type": "boolean"
+                    },
+                    "draft": {
+                      "type": "boolean"
+                    },
+                    "flagged": {
+                      "type": "boolean"
+                    },
+                    "seen": {
+                      "type": "boolean"
+                    }
+                  },
+                  "required": [
+                    "seen",
+                    "flagged",
+                    "draft",
+                    "answered"
+                  ],
+                  "type": "object"
+                },
+                "folder_ids": {
+                  "items": {
+                    "type": "string"
+                  },
+                  "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"
+                      ]
+                    }
+                  ]
+                },
+                "has_attachments": {
+                  "type": "boolean"
+                },
+                "id": {
+                  "description": "Message ID",
+                  "type": "string"
+                },
+                "keywords": {
+                  "description": "Active message keywords, including system flags and custom labels.",
+                  "example": [
+                    "$seen",
+                    "$flagged",
+                    "agent_triaged"
+                  ],
+                  "items": {
+                    "type": "string"
+                  },
+                  "type": "array"
+                },
+                "preview": {
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "received_at": {
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "sent_at": {
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "size_bytes": {
+                  "type": [
+                    "integer",
+                    "null"
+                  ]
+                },
+                "subject": {
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "thread_id": {
+                  "type": [
+                    "string",
+                    "null"
+                  ]
+                },
+                "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": [
+                "id",
+                "thread_id",
+                "folder_ids",
+                "subject",
+                "preview",
+                "from",
+                "to",
+                "cc",
+                "bcc",
+                "received_at",
+                "sent_at",
+                "size_bytes",
+                "has_attachments",
+                "keywords",
+                "flags"
+              ],
+              "type": "object"
+            },
+            "type": "array"
+          },
+          "meta": {
+            "additionalProperties": false,
+            "properties": {
+              "request_id": {
+                "type": "string"
+              },
+              "sync_state": {
+                "type": "string"
+              },
+              "thread_id": {
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id",
+              "thread_id"
+            ],
+            "type": "object"
+          },
+          "pagination": {
+            "additionalProperties": false,
+            "properties": {
+              "has_more": {
+                "type": "boolean"
+              },
+              "next_cursor": {
+                "type": "string"
+              }
+            },
+            "required": [
+              "has_more"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data",
+          "pagination"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "MailboxThreadMessageSummaryCursorListResponse"
+  }
+}

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

First observedv1.0.0Jul 13, 2026

TDQS

A3.8/5.0

Behavior3/5

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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false, so the safety profile is covered. The description adds only the known-thread scoping and the body-tool boundary, with no additional behavioral detail such as snippet-only results or pagination behavior. This is adequate but not enriched.

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 short, direct sentences with no filler. The primary usage instruction comes first, followed by the one relevant alternative-tool caution. Every sentence earns its place.

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?

With an output schema present and full parameter documentation in the schema, the description does not need to explain return shapes or parameter formats. It provides enough context for the core decision: list messages in a known thread, and defer full-content retrieval to body tools. A brief note on result format or when mailbox_id matters would elevate it further.

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 all five parameters are already documented in the schema. The description adds no further parameter-level meaning beyond restating that the operation is scoped to a known thread, which keeps it at baseline.

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

Purpose4/5

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

The description states a specific action and resource: 'list messages inside a known thread.' It is clear and distinct from the message-body siblings, but it does not explicitly differentiate itself from nearby list tools like mailbox_list_messages or mailbox_list_threads.

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 description gives a concrete usage condition: use this when the thread is already known. It also draws a clear boundary against body-content tools by saying they should only be used when full content is needed, which helps route the agent correctly.

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