Jsmtpd is a small and modular SMTP server daemon.
It is heavily based on plugins, and currently
supports spam detection (Spamassassin, Jasen, RBL
servers, and BlackList), anti-virus (Clam
Antivirus), Recipient rewriting, and many other
filters. Filtering is done with a customisable
tree of filters. Almost all functionality is implemented as plugins, making it easy to fit your needs.