mail server network protocol front-ends

 mailfront is a set of customizable network front-ends for mail servers.
 It contains complete SMTP, QMQP, QMTP, and POP3 front-ends as well as an
 authentication module for IMAP. The mail delivery front-ends also contain
 internal address filtering features.
 .
 Two SMTP back-ends are provided. One delivers mail to qmail-queue,
 mimicking most of the behavior of qmail-smtpd, with the addition of
 support for SMTP AUTH. The other rejects all SMTP commands if $SMTPREJECT
 is set, and execs its command line otherwise (in order to run the above
 program).