Shutting down

The web site catalog of mail filters & developer documentation and associated milter-announce mailing list will be permanently shut down on May 15, 2015. Thank you for your patronage over the last eight years.

Welcome to

This web site is designed to provide you with the world's most complete resource for open source and commercial Milters. You now have access to the interactive catalog of available Sendmail mail filters, or 'Milters'. In a single location, we have brought together the leading Developer Resources, API reference materials, and catalogs for building, posting and even rating Milters.

Searching for Milters has never been easier. Simply go to the Find a Milter column on the left and select your preferred Milter Categories and Licenses. Next, select the type of Sort search you would like perform and go!

In addition to the Find a Milter section, the primary navigation bar gives you complete access to the Milter Catalog, and Developer Center resources, plus you can now Add your own Milter to the catalog! Don't forget to visit the Lists section to sign up to receive our popular libmilter mailing list.

What exactly is a Milter?

With the growth in email volumes and the prevalence of threats like spam, viruses and targeted attacks (Denial of Service, Directory Attacks), the need arose to quickly and easily expand the Sendmail solution to include threat protection and optimized messaging performance.

The result was the Sendmail Content Management API (Sendmail Milter). This API enables third-party programs to access mail messages as they are being processed by the Mail Transfer Agent (MTA). This allows them to examine and modify message content and meta-information during the SMTP transaction. Any of these filters can be added or changed independently of other existing Milters.

The Milter API can be used on any OS that supports threads and it was created with the following application goals in mind:

  • Enhanced Safety and Security
  • Enhanced Reliability
  • Improved Simplicity
  • Improved Performance

The Milter API seeks to address site-wide filtering concerns in a scalable way by supporting the following MTAs:

Milter Development Kit

The Milter API provides an interface for third-party software to validate and modify messages as they pass through the mail transport system. The MTA configuration file specifies which filters are to be applied, and in what order, allowing an administrator to combine multiple independently-developed filters.

To find out more, click on the Developers link. Once you've written your own filter, you can add it to the catalog using the Add link, and even submit it for rating by other users and developers!