Internal Program Version: sendmail  8.18.1

Sendmail's Content Management API (milter) provides third-party programs to
access mail messages as they are being processed by the Mail Transfer Agent
(MTA), allowing them to examine and modify message content and
meta-information. Filtering policies implemented by Milter-conformant filters
may then be centrally configured and composed in an end-user's MTA
configuration file.

http://www.sendmail.org