Add support for XCLIENT on server and client side

[dejanstrbac]

Adding support for XCLIENT extension useful for proxying

https://www.postfix.org/XCLIENT_README.html

[endreszabo]

As a side note, I use the industry-standard haproxy's Proxy Protocol to preserve the client's IP (and more) with go-smtp.
Implementing support for the Proxy Protocol needs no change to go-smtp codebase and is very easy to roll out:

proxyListener := &proxyproto.Listener{
        Listener:   l,
}
defer proxyListener.Close()

if err := s.Serve(proxyListener); err != nil {
        log.Fatal(err)
}

[dejanstrbac]

As a side note, I use the industry-standard haproxy's Proxy Protocol to preserve the client's IP (and more) with go-smtp. Implementing support for the Proxy Protocol needs no change to go-smtp codebase and is very easy to roll out:

proxyListener := &proxyproto.Listener{
        Listener:   l,
}
defer proxyListener.Close()

if err := s.Serve(proxyListener); err != nil {
        log.Fatal(err)
}

I am aware of PROXY protocol, but Dovecot Proxy for LMTP delivery relies on XCLIENT extension which is our use case

[endreszabo]

but Dovecot Proxy for LMTP delivery

aah I see. I also use dovecot and was not aware of the existence of its LMPT server for delivery. Thus, I made a daemon that listens for gRPC calls from go-smtp and delivers the mails via the dovelot-lda. Not sure which one is better at this point :)

What would you say, what are the benefits for XCLIENT support with LMTP? Logging the original IP address?

[MarkNLD]

Watching this! Thanks for the work.