Amavis

Amavis is an open source content filter for electronic mail, implementing mail message transfer, decoding, some processing and checking, and interfacing with external content filters to provide protection against spam, viruses and other malware. It can be considered an interface between a mailer (MTA, Mail Transfer Agent) and one or more content filters.

Amavis can be used to:

Notable features:

A common mail filtering installation with Amavis consists of a Postfix as an MTA, SpamAssassin as a spam classifier, and ClamAV as an anti-virus protection, all running under a Unix-like operating system. Many other virus scanners (about 30) and some other spam scanners (CRM114, DSPAM, Bogofilter) are supported too, as well as some other MTAs.

Three topologies for interfacing with an MTA are supported. The amavisd process can be sandwiched between two instances of an MTA, yielding a classical after-queue mail filtering setup, or amavisd can be used as an SMTP proxy filter in a before-queue filtering setup, or the amavisd process can be consulted to provide mail classification but not to forward a mail message by itself, in which case the consulting client remains in charge of mail forwarding. This last approach is used in a Milter setup (with some limitations), or with a historical client program amavisd-submit.

Since version 2.7.0 a before-queue setup is preferred, as it allows for a mail message transfer to be rejected during an SMTP session with a sending client. In an after-queue setup filtering takes place after a mail message has already been received and enqueued by an MTA, in which case a mail filter can no longer reject a message, but can only deliver it (possibly tagged), or discard it, or generate a non-delivery notification, which can cause unwanted backscatter in case of bouncing a message with a fake sender address.