I've been working on securing my postfix configuration to enforce certificate validation and encryption on some known, higher-volume, or more sensitive connections between SMTP servers (port 25).
On many of the connections I've setup for secure transport there have been no problems (assuming proper TLS certificates are used). Unfortunately Gmail™ has been a problem. Sometimes it verifies and validates the certificate and other times it doesn't... for days.
After conferring with Google Security I believe I've come up with a solution. In my tls_policy file I've added the following:
gmail.com secure match=.google.com:google.com ciphers=high protocols=TLSv1.2
So far this is working[STRIKEOUT:but I'll continue to test].
If you run your own SMTP server and wish to maintain a secure connection with Gmail this is an easy way to enforce encryption as well as validate the certificate. Of course this doesn't protect the message while it's being stored on the server or workstation (or on Google's internal network). To protect messages at rest (on a server) one should use GPG or S/MIME. Using both TLS over the network between servers and GPG or S/MIME is beneficial to provide protection of the messages going over the Internet.
google.com secure ciphers=high protocols=TLSv1.2 comcast.net secure ciphers=high protocols=TLSv1.2 verizon.net secure ciphers=high protocols=TLSv1.2 hotmail.com secure ciphers=high protocols=TLSv1.2