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Email Sending MCP

Email Sending MCP

by resend
OverviewSchemaSourceRelated ServersDiscussionsAdmin
TypeScript
Local

Send Inbox Draft

send-inbox-draft
Destructive

Sends a Resend inbox draft as a real email after confirming its subject and recipients with the user; sending cannot be undone.

Instructions

Send a draft from a Resend inbox. This puts a real email in the mail the moment you call it, from the inbox's address to the recipients the draft already carries, and a sent email CANNOT be recalled or unsent. Before using this tool, you MUST double-check with the user that they want to send this draft. Reference the SUBJECT and the RECIPIENTS of the draft when double-checking, and warn the user that sending cannot be undone. You may only use this tool if the user explicitly confirms they want to send the draft after you double-check. The draft must have at least one "to" recipient and an html or text body, or the send fails. A standalone draft also needs a subject. A reply draft with an empty subject is sent with the thread's subject, prefixed with "Re:" when it does not already start with it. The whole email, encoded, can be at most 40 MB. Call get-inbox-draft first if you need to read back exactly what will go out, and update-inbox-draft to change it before sending. Get the draft ID from list-inbox-drafts. Returns the sent email's id.

Input Schema

NameRequiredDescriptionDefault
draftIdYesDraft ID. Use list-inbox-drafts to find draft IDs.
headersNoOptional custom email headers as key/value pairs (e.g. {"X-Entity-Ref-ID": "unique-id"}). Header names cannot be empty, and names and values cannot contain line breaks or null characters. In-Reply-To, References and Reply-To are reserved: Resend sets them, and passing any of them, in any letter case, fails.
inboxIdYesInbox ID, or the inbox email address (e.g. "support@example.com"). Not the Resend receiving address.
idempotencyKeyNoOptional unique key that prevents duplicate sends on retries (sent as the Idempotency-Key header). Use the same key when retrying the same logical email; use a new key for a different email. Must be 1-256 characters.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed1 schema field changedv2.27.1
    • changedInput schema / properties / inboxId / description
      Previous value: -"Inbox ID"New value: +"Inbox ID, or the inbox email address (e.g. \"support@example.com\"). Not the Resend receiving address."
  2. Addedv2.26.1

TDQS

A4.6/5.0
Behavior5/5

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

Annotations already flag destructiveHint=true and openWorldHint=true, and the description adds substantial context beyond them: the email is real and irreversible, it cannot be recalled or unsent, requires user confirmation, and has draft-content constraints (at least one 'to', html/text body, subject rules) plus a 40 MB cap. This is far richer than the annotations alone.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The irreversible-action warning is front-loaded and every sentence carries operational weight. It is slightly repetitive in restating the double-check/confirmation requirement twice, but the length is justified for a destructive send.

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

Completeness5/5

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

With no output schema, the description compensates by stating the return value ('Returns the sent email's id') and covers confirmation requirements, draft validity rules, subject handling, size limits, and related tools. Nothing needed to call it correctly is missing.

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 draftId, inboxId, headers, and idempotencyKey are already fully documented, including the reserved-header rule and idempotency guidance. The description only restates where to obtain the draft ID (list-inbox-drafts), adding no new parameter-level meaning, so the baseline of 3 applies.

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?

States a specific verb+resource ('Send a draft from a Resend inbox') and clearly distinguishes the action from siblings like send-new-inbox-email and send-broadcast. An agent can identify the operation and its target without opening the schema.

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 mandates double-checking with the user, warns the send cannot be undone, requires explicit confirmation, and routes to get-inbox-draft/update-inbox-draft/list-inbox-drafts as alternatives for reading, editing, or finding the draft. This is a textbook when/when-not/alternatives statement.

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

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