Fastmail MCP Server

reply_email

Reply to an existing email, automatically setting threading headers and determining recipients. Handles reply-all with optional sender identity detection.

Instructions

Reply to an existing email. Automatically sets threading headers (In-Reply-To, References) and determines recipients.

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": {
      "emailId": "<emailId>",
      "body": "<body>"
    },
    "name": "reply_email"
  }
}

Input Schema

Name	Required	Description
`body`	Yes	Reply body text (plain text)
`emailId`	Yes	ID of the email to reply to
`replyAll`	No	Reply to all recipients (default: reply only to sender)
`identityId`	No	Sender identity ID. Auto-detected if omitted.

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.3.0Aug 2, 2026

TDQS

A4.2/5.0

Behavior4/5

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

With no annotations, the description carries the transparency burden. It discloses two meaningful auto-behaviors: setting threading headers and determining recipients. However, it does not clarify whether the reply is sent immediately or created as a draft, nor mention response behavior, so it is not fully transparent.

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?

The description is two concise sentences, front-loaded with the core purpose ('Reply to an existing email'), and every clause earns its place by adding behavioral context without 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?

Given no output schema and no annotations, the description covers the essential purpose and key automatic behaviors. It could mention return format or note that it sends immediately, but for a reply tool with a straightforward action, the description is sufficiently complete for an AI agent.

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?

All four parameters have schema descriptions, so baseline is 3. The tool description adds no parameter-specific meaning beyond what the schema already states; the mention of auto recipient determination relates to overall behavior rather than clarifying individual params.

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 ('Reply') and resource ('existing email'), and adds key differentiators ('Automatically sets threading headers... determines recipients'), which clearly distinguishes it from sibling tools like send_email, forward_email, and create_draft.

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 implies usage for replying to existing emails and highlights automatic header/recipient handling, giving context for when this tool is appropriate. However, it does not explicitly name alternatives or state when not to use it, like the high-bar calibration example.

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