courier-mta binary package in Ubuntu Xenial amd64

 The Courier mail transfer agent (MTA) is an integrated mail/groupware
 server based on open commodity protocols, such as ESMTP, IMAP, POP3, LDAP,
 SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, webmail, and mailing list
 services within a single, consistent, framework.
 .
 This package provides an ESMTP daemon which uses the Maildir format
 as native storage format, supports IPv6, implements SMTP extensions
 for mailing list management and features integrated mail filtering.
 It can function as an intermediate mail relay, relaying mail
 between an internal LAN and the Internet, or perform final delivery
 to mailboxes.

Source package