milter

A milter is an extension to sendmail (MTA) that allows administrators to add mail filters for filtering spam, viruses or applying mail policy.

Generally, "milter" refers to an extension to the widely used open source mail transfer agent (MTA) sendmail that allows administrators to add mail filters for filtering spam, viruses or applying mail policy. In the PureMessage documentation, however, this concept has been extended to define the PureMessage filtering process(es) that operate with any supported MTA.