Agent Mail Gateway

Send a new email

send_message

Sends a new email written in Markdown as HTML and plain text, with optional attachments, after checking each recipient against allow_send_to and gateway review. Returns message_id and warnings.

Instructions

Send a new email written in Markdown (sent as HTML plus plain text), optionally with attachments. Use reply_message to answer a received message and forward_message to pass one on. Every recipient must be on allow_send_to; the gateway reviews the mail first and may reject it with review_rejected and reasons. Counts towards the hourly send limit and is saved to Sent. Returns { message_id, warnings }.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "to": "<to>",
      "subject": "<subject>",
      "body_markdown": "<body_markdown>"
    },
    "name": "send_message"
  }
}

Input Schema

Name	Required	Description
`cc`	No	Copy recipients; each must be on allow_send_to
`to`	Yes	Recipients; every address must be on allow_send_to
`bcc`	No	Hidden recipients; each must be on allow_send_to
`subject`	Yes	Subject line
`attachments`	No	Files to attach: text, base64 or a file from a received message
`reply_to_id`	No	Message id this answers (threading only; prefer reply_message, which fills in recipients)
`body_markdown`	Yes	Message body in Markdown

Schema Changelog

Changes observed during successful MCP inspections.

Changed14 schema fields changedv0.3.2Oct 3, 2026

addedInput schema / properties / attachments / description

Added value: +"Files to attach: text, base64 or a file from a received message"

addedInput schema / properties / attachments / items / properties / content_base64 / description
```
Added value: +"File content as base64 (for binary files)"
```

addedInput schema / properties / attachments / items / properties / content_text

Added value: +{
+  "description": "File content as plain text (for txt, csv, md, json, ics, ...)",
+  "type": "string"
+}

removedInput schema / properties / attachments / items / properties / content_type / default
```
Removed value: -"application/octet-stream"
```
addedInput schema / properties / attachments / items / properties / content_type / description
```
Added value: +"MIME type, e.g. text/csv or application/pdf"
```
addedInput schema / properties / attachments / items / properties / filename / description
```
Added value: +"File name; optional with from_message (the original name is used)"
```

addedInput schema / properties / attachments / items / properties / from_message

Added value: +{
+  "description": "Attach a file from a received message without downloading it",
+  "properties": {
+    "id": {
+      "description": "Message id the attachment belongs to",
+      "type": "string"
+    },
+    "index": {
+      "description": "Attachment index from read_message.attachments",
+      "maximum": 9007199254740991,
+      "minimum": 0,
+      "type": "integer"
+    }
+  },
+  "required": [
+    "id",
+    "index"
+  ],
+  "type": "object"
+}

removedInput schema / properties / attachments / items / required
```
Removed value: -[
-  "filename",
-  "content_base64"
-]
```
addedInput schema / properties / attachments / maxItems
```
Added value: +20
```

addedInput schema / properties / bcc / description

Added value: +"Hidden recipients; each must be on allow_send_to"

addedInput schema / properties / body_markdown / maxLength
```
Added value: +512000
```

addedInput schema / properties / cc / description

Added value: +"Copy recipients; each must be on allow_send_to"

changedInput schema / properties / reply_to_id / description

Previous value: -"id of a received message this replies to"New value: +"Message id this answers (threading only; prefer reply_message, which fills in recipients)"

addedInput schema / properties / subject / description
```
Added value: +"Subject line"
```

First observedv0.1.0Oct 2, 2026

TDQS

A4.4/5.0

Behavior4/5

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

Annotations declare readOnlyHint=false, openWorldHint=true, idempotentHint=false, and destructiveHint=false. The description adds valuable context beyond this: gateway review may reject with review_rejected and reasons, counts towards hourly send limit, saved to Sent, and returns { message_id, warnings }. It doesn't mention whether attachments are scanned or if review is asynchronous, but the disclosure is rich for a tool with 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, front-loaded with purpose, then sibling routing, then behavioral constraints. Every sentence adds value; no redundancy.

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?

The description covers purpose, alternatives, allow_send_to precondition, review rejection, rate limit, storage, and return shape. With no output schema, it correctly includes the return object. However, it doesn't mention max attachment count (20) or body size limit (512000) – though those are in the schema. Minor gaps remain but overall complete.

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 already documents all 7 parameters in detail. The description adds no parameter-specific syntax or format information beyond what the schema provides. Baseline 3 is appropriate when the schema does the heavy lifting.

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 verb+resource ('Send a new email') and immediately specifies the format ('written in Markdown (sent as HTML plus plain text)'). It distinguishes itself from siblings 'reply_message' and 'forward_message' by name, so an agent can pick the right tool without opening schemas.

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

Usage Guidelines5/5

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

Explicitly states when to use alternatives: 'Use reply_message to answer a received message and forward_message to pass one on.' It also sets a precondition: 'Every recipient must be on allow_send_to.' This is clear when-to-use and when-not-to-use guidance.

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

Deploy Server