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 and content.

libmilter provides the implementation of this API.

Note that although the API of this library is fairly stable and
will most likely only see additions, the interface between this
library and sendmail is subject to change without notice.

Homepage:
http://www.sendmail.org/
