Everything associated with mail has High rating as we are dealing with private and restricted data. So no one else then the owner of the mail should have access. Therefore, mail is secured through the user authentication and authorization measures of the OS user account system. For the secure protocol imaps is used. For WEBmail https is in place.

There are two elements to the mail system: Inbox and personal folders. The Inbox is physically located at the mail server to which no user has login rights. Inbox content can only be accessed after successful authentication. The personal mail folder structure resides inside the user home directory space. Both are accessed using the dovecot or MS Exchange utility (based on imaps encrypted and authenticated protocol).

Sending mail is performed using the submission protocol which is both secure encrypted and authenticated.

See here how to setup the mail client or access WEBmail.