Sendmail::PMilter provides access to the message processing hooks in a
running Mail Transfer Agent (Sendmail or Postfix), so that filters
('milters') which process mail can be written entirely in Perl.  Mail
processing according to SMTP takes place at a number of well-defined
stages ('connect', 'helo', etc.) during the exchange of the message
between client and server.  At each stage, the sets of data available
(the connecting IP, sender address etc.) and the permissible actions
(rewrite a message header, ACCEPT a message, REJECT it etc.) are also
well-defined and are to some extent configurable.