Sendmux Email Inbox API + Sending

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Send Mailbox Message

mailbox_send_message

Send an email from the authenticated mailbox with custom recipients, subject, HTML/plain text bodies, and attachments. Supports safe retries via Idempotency-Key.

Instructions

Use this to send a message from the authenticated mailbox. For attachments, call mailbox_upload_attachment first using presign_upload_url or tiny content_base64, then pass the returned blob_id. Include an Idempotency-Key for retries.

Input Schema

TableJSON Schema

Name	Required	Description
`cc`	No	CC recipients.
`to`	No	Primary recipients.
`bcc`	No	BCC recipients.
`from`	No	Sender address. Defaults to mailbox identity.
`subject`	No	Subject line for the outgoing email.
`reply_to`	No	Reply-To recipients.
`html_body`	No	HTML body.
`text_body`	No	Plain text body.
`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.
`attachments`	No	Attachments to send with the message.
`custom_headers`	No	Custom headers to include.
`Idempotency-Key`	No	Client-chosen unique key to safely retry the request. Cached for 24h per (mailbox, endpoint, key). Different body with same key returns 409 idempotency_conflict.

Output Schema

TableJSON Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

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": {
+            "additionalProperties": false,
+            "properties": {
+              "message_id": {
+                "type": "string"
+              },
+              "status": {
+                "enum": [
+                  "queued"
+                ],
+                "type": "string"
+              }
+            },
+            "required": [
+              "message_id",
+              "status"
+            ],
+            "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": "MailboxSendResultResponse"
+  }
+}

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

First observedv1.0.0Jul 13, 2026

TDQS

A3.7/5.0

Behavior3/5

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

Annotations already mark this as a mutating operation (readOnlyHint=false), so the bar is lower. The description adds useful context beyond the annotations: the dependency on a prior upload call, and the retry-with-key recommendation. It does not disclose the 24h idempotency cache or 409 conflict behavior, but those already live in the schema, so no additional credit is needed.

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 tight sentences with zero filler. The primary purpose is front-loaded, followed by the two most important operational directives (attachment workflow and idempotency). Every sentence earns its place.

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

Completeness3/5

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

For a 12-parameter tool with nested objects, the schema and annotations carry most of the load, and the description supplies the key workflow sequencing. The notable gap is sibling disambiguation: with sending_send_email and sending_send_email_batch in the same toolset, an agent gets no guidance on which sending path to choose. The openWorldHint=true signal also goes unaddressed in the description.

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 schema fully documents all 12 parameters, and the baseline is 3. The description reinforces the attachment flow (upload first, pass blob_id) and the Idempotency-Key usage, but adds little meaning beyond what the parameter descriptions already state.

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 clearly states a specific verb and resource: 'send a message from the authenticated mailbox.' It is unambiguous about the core operation. However, it does not differentiate itself from the sibling tools sending_send_email and sending_send_email_batch, which also send email, leaving the agent to guess which one to pick.

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 concrete usage context: it prescribes a specific pre-requisite workflow for attachments ('call mailbox_upload_attachment first... then pass the returned blob_id') and instructs the agent to 'Include an Idempotency-Key for retries.' It does not, however, state when not to use this tool or mention the sending_send_email alternatives.

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