MIMEDefang is a program for inspecting and modifying e-mail messages as
they pass through your mail relay. MIMEDefang is written in Perl, and its
filter actions are expressed in Perl, so it's highly flexible. Here are some
things that you can do very easily with MIMEDefang: 

    Delete or alter attachments based on file name, contents, results of a
    virus scan, attachment size, etc. 

    Replace large attachments with links to a centrally-stored copy to ease
    the burden on POP3 users with slow modem links. 

    Add boilerplate text to e-mail messages. 

    Customize filter rules based on domain, user-name, relay machine, etc. 

    Reject unacceptable messages, where you define what "unacceptable" means. 

    Add or delete recipients for a message. 

WWW: http://www.mimedefang.org/