Sendmux Email Inbox API + Sending

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by Sendmux

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Hybrid

Send Email Batch

sending_send_email_batch

Batch-send multiple outbound emails in one request, ensuring each recipient and message is confirmed before delivery.

Instructions

Use this to send multiple outbound emails in one request. For attachments, prefer uploaded attachment_id refs; avoid inline base64 except for tiny generated content. Use it only when the user confirms every recipient and message.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`messages`	Yes	Array of email messages to send (max 100)
`Idempotency-Key`	No	Optional client-generated key to make the request idempotent for 24 hours. Replays under the same key return the cached response; a reused key with a different body returns 409 idempotency_conflict.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedv1.0.9Sep 16, 2026

addedInput schema / properties / messages / items / properties / delivery_group

Added value: +{
+  "anyOf": [
+    {
+      "pattern": "^dgrp_[a-z0-9][a-z0-9_-]{0,122}$",
+      "type": "string"
+    },
+    {
+      "items": {
+        "pattern": "^dgrp_[a-z0-9][a-z0-9_-]{0,122}$",
+        "type": "string"
+      },
+      "maxItems": 50,
+      "minItems": 1,
+      "type": "array"
+    }
+  ],
+  "description": "One delivery group public ID or a non-empty array of up to 50 IDs"
+}

Changed5 schema fields changedv1.0.8Sep 15, 2026

addedInput schema / properties / Idempotency-Key / example
```
Added value: +"order-12345-2026-04-24"
```

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": false,
+            "properties": {
+              "request_id": {
+                "description": "Unique request identifier",
+                "example": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx",
+                "type": "string"
+              }
+            },
+            "required": [
+              "request_id"
+            ],
+            "type": "object"
+          },
+          "ok": {
+            "enum": [
+              true
+            ],
+            "type": "boolean"
+          }
+        },
+        "required": [
+          "ok",
+          "meta"
+        ],
+        "type": "object"
+      },
+      {
+        "properties": {
+          "data": {
+            "additionalProperties": false,
+            "properties": {
+              "results": {
+                "items": {
+                  "additionalProperties": false,
+                  "properties": {
+                    "error": {
+                      "allOf": [
+                        {
+                          "additionalProperties": false,
+                          "properties": {
+                            "code": {
+                              "description": "Machine-readable error code",
+                              "example": "validation_error",
+                              "type": "string"
+                            },
+                            "doc_url": {
+                              "description": "Link to error documentation",
+                              "format": "uri",
+                              "type": "string"
+                            },
+                            "errors": {
+                              "description": "Accumulated per-field issues (validation errors)",
+                              "items": {
+                                "additionalProperties": false,
+                                "properties": {
+                                  "code": {
+                                    "description": "Issue code from the validator",
+                                    "example": "invalid_string",
+                                    "type": "string"
+                                  },
+                                  "field": {
+                                    "description": "Dot-path to the failing field",
+                                    "example": "to.email",
+                                    "type": "string"
+                                  },
+                                  "message": {
+                                    "description": "Human-readable issue message",
+                                    "type": "string"
+                                  }
+                                },
+                                "required": [
+                                  "field",
+                                  "code",
+                                  "message"
+                                ],
+                                "type": "object"
+                              },
+                              "type": "array"
+                            },
+                            "message": {
+                              "description": "Human-readable error description",
+                              "type": "string"
+                            },
+                            "param": {
+                              "description": "Parameter that caused the error",
+                              "type": "string"
+                            },
+                            "retryable": {
+                              "description": "True when the client may safely retry the same request. 429 and 5xx default true; 4xx default false.",
+                              "type": "boolean"
+                            }
+                          },
+                          "required": [
+                            "code",
+                            "message",
+                            "retryable"
+                          ],
+                          "type": "object"
+                        },
+                        {
+                          "description": "Error details (only if failed)"
+                        }
+                      ]
+                    },
+                    "index": {
+                      "description": "Position in the input array",
+                      "type": "integer"
+                    },
+                    "message_id": {
+                      "description": "Message ID (null if failed)",
+                      "pattern": "^eml_[a-z0-9]{24}$",
+                      "type": [
+                        "string",
+                        "null"
+                      ]
+                    },
+                    "status": {
+                      "description": "Result status",
+                      "enum": [
+                        "queued",
+                        "failed"
+                      ],
+                      "type": "string"
+                    }
+                  },
+                  "required": [
+                    "index",
+                    "message_id",
+                    "status"
+                  ],
+                  "type": "object"
+                },
+                "type": "array"
+              },
+              "summary": {
+                "additionalProperties": false,
+                "properties": {
+                  "failed": {
+                    "description": "Failed count",
+                    "type": "integer"
+                  },
+                  "queued": {
+                    "description": "Successfully queued count",
+                    "type": "integer"
+                  },
+                  "total": {
+                    "description": "Total messages in batch",
+                    "type": "integer"
+                  }
+                },
+                "required": [
+                  "total",
+                  "queued",
+                  "failed"
+                ],
+                "type": "object"
+              }
+            },
+            "required": [
+              "results",
+              "summary"
+            ],
+            "type": "object"
+          }
+        },
+        "required": [
+          "data"
+        ],
+        "type": "object"
+      }
+    ],
+    "unevaluatedProperties": {
+      "not": {}
+    },
+    "x-fastmcp-top-level-schema": "BatchSendSuccessResponse"
+  }
+}

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

Changed4 schema fields changedv1.0.1Aug 12, 2026
- changedInput schema / properties / messages / items / properties / bcc / description
```
Previous value: -"BCC recipients (max 100)"New value: +"BCC recipients (subject to 50 total To, CC, and BCC recipients)"
```
- changedInput schema / properties / messages / items / properties / bcc / maxItems
```
Previous value: -100New value: +49
```
- changedInput schema / properties / messages / items / properties / cc / description
```
Previous value: -"CC recipients (max 100)"New value: +"CC recipients (subject to 50 total To, CC, and BCC recipients)"
```
- changedInput schema / properties / messages / items / properties / cc / maxItems
```
Previous value: -100New value: +49
```
First observedv1.0.0Jul 13, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Beyond annotations (readOnlyHint=false, openWorldHint=true), the description adds a safety gate requiring user confirmation of every recipient and message. It also advises against inline base64 attachments except for tiny generated content, which is useful behavioral context. It doesn't disclose partial-send or failure behavior, but the description meaningfully supplements the minimal annotations.

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?

Three sentences with no redundancy: the primary purpose is front-loaded, followed by a concise attachment tip and a safety condition. Every sentence contributes actionable information, and the structure is optimal for quick agent comprehension.

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 tool's complexity (a messages array with many subfields), the description combined with 100% schema coverage and an output schema is largely complete. It lacks an explicit comparison to the single-send sibling and doesn't mention batch size limits, but these are recoverable from the schema (max 100) and the tool name. The main gap is the absence of an explicit alternative, so 4 is appropriate.

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

Parameters4/5

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

Schema coverage is 100%, so the baseline is 3. The description adds specific parameter guidance by preferring uploaded attachment_id refs and discouraging inline base64 except for tiny generated content. This directly informs how to populate the attachments parameter, which is beyond the schema's generic 'Use attachment_id refs for uploaded files' note.

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 states a specific action: 'send multiple outbound emails in one request.' This clearly distinguishes the tool from the sibling sending_send_email by emphasizing 'multiple' and 'in one request.' The resource (emails) and operation (sending) are unambiguous.

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 clear prerequisite: 'Use it only when the user confirms every recipient and message.' It also implies this tool is for batch sending rather than single sends, though it doesn't explicitly name sending_send_email as the alternative. The attachment guidance offers practical context for when to use this tool.

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