courier-base binary package in Ubuntu Bionic amd64

 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.

Source package