MIMEDefang is an e-mail filter program which works with Sendmail 8.12
and later.  MIMEDefang filters all e-mail messages sent via SMTP.
MIMEDefang splits multi-part MIME messages into their components and
potentially deletes or modifies the various parts.  It then
reassembles the parts back into an e-mail message and sends it on its
way.

MIMEDefang is written (mostly) in Perl, and the filter actions are
expressed in Perl.  This makes MIMEDefang highly flexible and
configurable.  As a simple example, you can delete all *.exe and *.com
files, convert all Word documents to HTML, and allow other attachments
through.

MIMEDefang uses the "milter" feature of Sendmail to "listen in" to
SMTP connections.  It runs a scan once for each message, not once for
each recipient (as simpler procmail-based systems do.)  Therefore, it
is more CPU-friendly than procmail-based systems.  In addition,
because MIMEDefang can participate in the SMTP connection, you can
bounce messages (something impossible to do with procmail-based
systems.)