Mailfront is a package containing 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).