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.