amavisd-new is a high-performance interface between mailer (MTA) and
content checkers: virus scanners, and/or SpamAssassin. It is written in
Perl for maintainability, without paying a significant price for speed.
It talks to MTA via (E)SMTP or LMTP, or by using helper programs. Best
with Postfix, fine with dual-sendmail setup and Exim v4, works with
sendmail/milter, or with any MTA as a SMTP relay. 'Howto' for qmail
available as well.
