Binary package “courier-mta-ssl” in ubuntu bionic

Courier mail server - ESMTP over TLS [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 package provided the SSL variant of the ESMTP daemon. It now is
 a transitional package and can be safely removed.