Sendmail Mail Filter API (Milter) The Sendmail Mail Filter API (Milter) is designed to allow third-party programs access to mail messages as they are being processed in order to filter meta-information (headers) and content. The Debian Sendmail package is built to support libmilter. This is the libmilter static library and include files for development. The documentation on building a Milter is in the sendmail-doc package.