@pvagner @erion it's correct that the current chatmail relays require port 25 to be reachable. There are some discussions around other server-to-server transports, reflected in https://github.com/chatmail/relay/issues/487 even if mostly talks about Tor. The email system is traditionally pretty flexible about transports, especially server-to-server ones. There are also some deployments that use uucp for long range radio based transporting between MTAs.