SSL is fully supported secure email on all protocols to ensure sensitive email messages or passwords cannot be read by hackers or users who may have access to the communication channel between you and the mail server. If you are running a mail server that doesn't support this feature then essentially anyone with access to your network can steal passwords. You can specify whether to allow users to login in non secure mode or not with IP range limits too. Almost all popular email clients now support SSL/TLS.
POP: Secure to regular port using STARTTLS, secure to dedicated port.
SMTP: Secure to regular port using STARTTLS, secure to dedicated port.
IMAP: Secure to regular port using STARTTLS, secure to dedicated port.
HTTPS: All web based administration tasks can be done either using secure HTTPS or standard HTTP.
Mirroring / replication: The in-built server mirroring feature mirrors the server over a secure link.