Foxy Mail Wire: an open format for compressed & encrypted email

Internet mail still moves as MIME over IMAP and SMTP. That’s fine for delivery — bad if you want smaller messages or real end-to-end privacy of the body, without locking the format inside one app.

Foxy Mail Wire is a small, open specification for that: compress and encrypt the MIME payload, leave normal envelope headers alone, and let any client implement it.

What’s in the box

• .foxyz / FOXYC1 — ZSTD-compressed message body
• .foxye / FOXYE1 — AES-256-GCM + X25519 encrypted body (ZSTD inside)

Compatible clients advertise support with simple headers (X-Foxy-Compress, X-Foxy-Encrypt, and when packing, X-Foxy-Pubkey). Non-aware clients still see a short plain-text notice plus an attachment — they just can’t open the payload.

This is not an IETF draft and not the Foxy Mail app. It’s a wire contract: framing, MIME wrapper, and crypto choices, so implementers don’t have to reverse-engineer a client.

Why publish it

Compression and E2E for mail only matter if more than one product can speak the same bytes. Putting the format on Codeberg keeps it easy to read, fork, and implement — without App Store binaries or a private wiki.

Spec

Full draft (CC0):

https://codeberg.org/DocJoJo/foxy-mail-wire

If you build a mail client (or a library), that’s the place to start. Feedback and independent implementations welcome.