Milter

From Grokdoc

milter is a facility built into the sendmail mailsystem to process mail at the server end. its used to run the SpamAssassin system amongst other things.

There are packages available to use milter to filter mail on sender, recepient, text in the body, pattern matches over mail. It can process the 'envelope' -the part of email which is not normally visible to the mail receiver in their mailbox.