Attachments

Outbound attachments use a two-step process:

1. Upload — POST a file to /v1/attachments as multipart form data. You get back an attachment ID.
2. Attach — Include the attachment ID(s) in the attachments array when sending a message via POST /v1/mailboxes/:id/messages.

Inbound attachments are extracted automatically by the JMAP poller: bytes are uploaded to R2, an Attachment row is linked to the inbound Message, and a fresh presigned download_url is included in every message.received webhook/SSE event. See Inbound attachments for the schema, the size/count caps, and the inline image rewrite example.

Fetch attachment metadata and a presigned download URL via GET /v1/attachments/:id. The presigned URL is valid for 24 hours and points directly to Cloudflare R2 storage. See the Attachments API reference.

• Outbound: max file size per attachment 25 MB; max total message size 40 MB (Resend limit).
• Inbound: max file size per attachment 25 MB; max 20 attachments per inbound message. Over-cap parts are dropped and the message row is flagged with attachmentsDropped: true.
• Storage counts toward your account's storage quota