Download the message as message/rfc822 with Content-Disposition attachment and filename {msgId}.eml. Quotes, CR, LF, other control characters, and path separators are removed from the filename. A non-ASCII name also sets filename* (RFC 5987 UTF-8). Responses set X-Content-Type-Options: nosniff, Content-Security-Policy: default-src 'none'; sandbox; frame-ancestors 'none', X-Frame-Options: DENY, and Referrer-Policy: no-referrer. Cache-Control is private. There is no Cross-Origin-Resource-Policy header. ?download=1 also forces attachment. Inbound mail returns the stored raw body. A raw body of 200,000 characters or more is stored as the first 200,000 characters and sets x-cooper-raw-truncated: 1. Inbound mail with no stored raw returns 404 raw_not_available. Outbound mail is rebuilt from stored fields and sets X-Cooper-Reconstructed: 1. A held message returns HTTP 409 message_held until release. Another account's message is 404.