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.
MIMEDefang Alternatives
MIMEDefang is described as 'GPL licensed framework for filtering e-mail. It uses sendmail's "Milter" API, some C glue code, and some Perl code to let the user write high-performance mail filters in Perl' and is an app. There are 1 alternatives to MIMEDefang for Linux. The best MIMEDefang alternative is amavisd-new, which is both free and Open Source.