Docs

Attachments

Send attachments as base64 and download received ones.

Sending

Add attachments to a send or reply body. Each item has filename, content_type and content encoded as base64. Attachments of one message can total up to 6 MB; larger requests return 413 payload_too_large.

Attachment in a send body

{
  "to": ["accounts@example.com"],
  "subject": "Invoice 2026-118",
  "text": "Invoice attached.",
  "attachments": [
    { "filename": "invoice-2026-118.pdf", "content_type": "application/pdf", "content": "JVBERi0xLjcK..." }
  ]
}

Receiving

A received message lists its attachments with attachment_id, filename, content_type and size. Download the bytes with GET .../messages/{message}/attachments/{attachment}; the response carries the original content type.

Over MCP, get_attachment returns text files as text and other files as base64, up to 5 MB. Download larger files with the REST endpoint.

Endpoints

Download an attachment

Parameters

Name	Type	Notes
`inbox`	string (email)	in path, required
`message`	string	in path, required, Message UUID or RFC Message-ID (URL encoded)
`attachment`	string (uuid)	in path, required
`include_review`	boolean	in query, Also return messages held for security review; needs message_review, default false

Responses

200 Attachment bytes with its content type

Attachment object