Mailbox API attachments

The Sendmux Mailbox API is for mailbox-scoped access. Use it when a client should act as a mailbox, or as one mailbox from a connected-app mailbox set, without managing team-wide resources.

Attachments

Use POST /mailbox/attachments:upload for binary files up to 7.5 MB. Send the raw bytes in the request body, then pass the returned blob_id to POST /mailbox/messages/send. Final outbound messages, including bodies, headers, and encoded attachments, must fit inside the 25 MB raw message limit.

curl -X POST "https://app.sendmux.ai/api/v1/mailbox/attachments:upload?filename=invoice.pdf" \
  -H "Authorization: Bearer smx_mbx_your_key_here" \
  -H "Content-Type: application/pdf" \
  --data-binary "@invoice.pdf"

{
  "ok": true,
  "data": {
    "blob_id": "blob_123",
    "filename": "invoice.pdf",
    "content_type": "application/pdf",
    "size_bytes": 12345
  },
  "meta": {
    "request_id": "req_clxxxxxxxxxxxxxxxxxxxxxxxxx"
  }
}

Then send the uploaded attachment by ID:

curl -X POST https://app.sendmux.ai/api/v1/mailbox/messages/send \
  -H "Authorization: Bearer smx_mbx_your_key_here" \
  -H "Content-Type: application/json" \
  -d '{
    "to": [{ "email": "customer@example.com" }],
    "subject": "Invoice",
    "text_body": "Attached is your invoice.",
    "attachments": [
      {
        "filename": "invoice.pdf",
        "content_type": "application/pdf",
        "blob_id": "blob_123"
      }
    ]
  }'

Small inline base64 attachments are still supported on messages/send, but the upload endpoint avoids large JSON request bodies. Message metadata can include a short-lived download_url for clients that need a no-header download link. Fetch it promptly without an Authorization header. If it expires, re-read message or attachment metadata to receive a fresh URL. MCP clients should prefer mailbox_read_attachment for text-like inbound attachments and use the returned link only for binary or oversized files. Attachment downloads support byte ranges:

curl "https://app.sendmux.ai/api/v1/mailbox/messages/msg_123/attachments/att_123" \
  -H "Authorization: Bearer smx_mbx_your_key_here" \
  -H "Range: bytes=0-1048575" \
  --output invoice.part

Getting started

Developer tools

AI integrations

Sending

Receiving

Domains

Monitoring

Webhooks

Use cases

Integrations

Account and billing

Mailbox API attachments

Attachments

Next steps

Mailbox API introduction

Synchronisation

​Attachments

​Next steps

Mailbox API introduction

Synchronisation

Attachments

Next steps