Milter.org is the largest open source resource center for the worlds leading mail filters (or Milters). This site includes a catalog of nearly 100 open source and commercial Milters.

The sendmail API "Milter" enables third-party programs to access mail messagesas they are being processed by the MTA. This allows them to examine and modify message content and meta-information during the SMTP transaction.

Milter developers are encouraged to contact Sendmail Inc. if you are interested in certifying a Milter application to run on the Sendmail Sentrion Message Processor platform and to add your application to the Sentrion Application Store.