SMTP and SSL/TLS

Question

If I understand correctly, communication between SMTP servers may be encrypted but is not required. Because it is optional, does it really have benefits? When server A connects to server B, both of which supports SSL/TLS, can't a MITM just jump in and say server B doesn't support TLS?

Edit: I'm referring to the SMTP connection say, between Gmail's SMTP server and my SMTP server, when sending an email from Gmail to @mydomain.com. Is there a purpose for Gmail to use SSL/TLS when communicating with my SMTP server? From my understanding, there isn't anything preventing a MITM attack there.

Answer 1

Opportunistic SSL will deter passive attackers who just spy on the line. It also slightly raises the bar for active attackers: in order to be able to alter the transmitted data (e.g. to inject a forged email in the stream), the attacker now must make a full MitM with fake certificate and decrypt/reencrypt the whole connection.

But, really, that's for public relations. Big mail relay hosts like Gmail use SSL/TLS when they can mostly to show that they are responsible and made an effort towards "more security". Ultimately, this is not a bad move: if the fashion takes hold and turns into tradition, then we may hope that at one time SSL/TLS will become mandatory. If the SMTP servers then decide to use it properly (i.e. with adequate validation of the peer certificate, complete with revocation checks), then overall security will have increased.

Answer 2

SMTP is inherently a relayed protocol, with any relay not belonging to the source or target domain being a man-in-the-middle. SMTP would approach confidentiality offered by HTTPS if a SMTP server controlled by your organisation were to look up a mail server from the MX DNS records of the target domain and then only transmit the email via TLS to that server.

Obviously, this would exclude all webmail - where the Javascript client delivers the mail via HTTPS to the webmail provider's HTTPS server, which then decrypts it and then stores/transfers it in clear-text to its SMTP servers and then may or may not send it over TLS to the target domain's SMTP server.