Xmodulo shows you how to secure a mail server using encryption

SSL (Secure Sockets Layer) and its descendant TLS (Transport Layer Security) are the most widely used protocols for encrypting data that is exchanged between a server and a client. These protocols often use X.509 certificates and asymmetric cryptography.

STARTTTLS is another method of securing plain-text communication. This protocol also encrypts data with SSL or TLS, but with the same port as the plain-text protocols, instead of using separate ports for SSL/TLS-encrypted communications. For example, IMAP over STARTTLS uses the same port as IMAP (143), while IMAPS (IMAP over SSL) uses a separate port 993.

  How to secure a mail server using encryption