Install courier-ssl by entering the following commands in the terminal:
sudo apt update sudo apt install courier-ssl
Courier mail server - TLS Support [transitional]
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP, TLS, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list services within a single, consistent, framework. . This is a transitional package and can be safely removed.