I hate to differ, mailq, but SSL between MTAs (that is, between your mail server and other mail servers) is perfectly well-supported and well-understood. It runs happily on port 25. When you connect to a mail server offering this, it's advertised in the EHLO phase:
[madhatta@anni ~]$ telnet www.teaparty.net 25
Connected to www.teaparty.net.
Escape character is '^]'.
220 : ESMTP you accept terms at http://www.teaparty.net/smtp.html
250-www.teaparty.net Hello 88-111-161-32.dynamic.dsl.as9105.com [18.104.22.168], pleased to meet you
A fellow mail server who's willing to talk TLS can then request escalation to encrypted communication, and the rest of the SMTP conversation can then happen under cover of crypto. The signed or unsigned state of a peer's certificate shows up in my sendmail logs thus:
Sep 25 22:42:05 www sendmail: STARTTLS=server, relay=nagios.teaparty.net [22.214.171.124], version=TLSv1/SSLv3, verify=NO, cipher=DHE-RSA-AES256-SHA, bits=256/256
In this case, I'm connecting to the foreign server (or it would say
STARTTLS=client) and I can't, using my certificate bundle, verify the peer's certificate (or it would say
verify=YES). But it's perfectly sound crypto, and worth doing.
Other than that I agree with your (otherwise excellent) answer.