This is a replacement SMTP daemon which installs alongside a mail delivery and transport system such as Exim, Postfix or Qmail, or used as an SMTP proxy for a remote/DMZ MTA.

The qpsmtpd damon emphasizes spam detection during the SMTP transaction, attempting to reach spam/nonspam decisions prior to accepting each message, thereby eliminating much bounce/forgery blowback. It exploits its visbility into the network transaction to detect certain behaviors often exhibited by spam sending agents.

Qpsmtpd is written in Perl, with an extensive plugin API making it easy to add new features or replace functional components.