courier-mta 0.78.0-2ubuntu2 (arm64 binary) in ubuntu bionic

 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 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.
 .
 ESMTP over TLS is handled by the regular ESMTP daemon in conjunction
 with the TLS wrapper.