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.

NOTE: With Sendmail 8.12.0 and above, the Milter API is built-in.
Before 8.12.0, both libmilter and the callouts in sendmail are marked 
as an FFR (For Future Release).  This means that support is *NOT* 
guaranteed - but things have been working well, and the usenet/mailing 
list (comp.mail.sendmail) does help if you have questions/problems.

Everything you'll need to create a milter filter is contained in
/usr/share/doc/sendmail (for which you need to install sendmail-doc):

  - libmilter/README  Document on build/install/config for milter
                      NOTE: see updated make in milter directory
  - cf.README         Document on configuring milter in sendmail.mc
  - RELEASE_NOTES     Any late breaking news

and /usr/share/sendmail/examples (for which you need to install sendmail):

  - examples/milter/*   Sample milter filter and makefile

-- 
Rick Nelson
cowboy@debian.org