Install courier-base by entering the following commands in the terminal:
sudo apt update sudo apt install courier-base
Courier mail server - base system
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 package provides the functionality needed by all courier packages such as some configuration files, helper programs and the Courier TCP server daemon. . Courier uses the a TLS wrapper application named couriertls instead of embedding TLS support in the different applications. Additionally, this package contains a default set of trusted X.509 root CA certs.