Connections between mail servers have traditionally been very weakly secured. STARTTLS is an extension to add connection security to existing protocols. Using only STARTTLS to secure connections between mail servers helps against so-called passive attackers. An active attacker can easily undo the use of STARTTLS. You can use DANE to reliably communicate that your mail servers can be accessed via a secure connection.
The government Web guidelines apply STARTTLS and DANE for e-mail traffic, as an addition to the pass-or-leave list of open standards. View the Fact Sheet Secure Mail Server Connections here.
