# SMTP extensions

Bongo keeps the classic `HELO`/`MAIL`/`RCPT`/`DATA` path for old clients and
advertises ESMTP extensions after `EHLO`. The server advertises `8BITMIME` and
`SMTPUTF8` only when the corresponding configuration is enabled. `SMTPUTF8`
is accepted as a `MAIL FROM` parameter only after `EHLO`; non-ASCII envelope
addresses require it and are validated as UTF-8.

`BODY=8BITMIME` is recorded with the queued message and is sent to a remote
peer only when that peer advertises `8BITMIME`. Bongo does not implement
`BDAT`/`CHUNKING`, so `BODY=BINARYMIME` is rejected explicitly instead of
silently treating binary data as a normal `DATA` message.

Outgoing SMTP uses the peer's advertised `SIZE`, `8BITMIME` and `SMTPUTF8`
capabilities. Internationalized domains are converted to their DNS
ASCII-compatible form for MX lookup while the original UTF-8 address remains
in the SMTP envelope. External SMTP relays use libcurl and receive the same
`SMTPUTF8`/`BODY=8BITMIME` envelope parameters.

Relevant standards are [RFC 6531](https://www.rfc-editor.org/rfc/rfc6531.html),
[RFC 3030](https://www.rfc-editor.org/rfc/rfc3030.html) and
[RFC 3461](https://www.rfc-editor.org/rfc/rfc3461.html).