MIMEDefang is an e-mail filter program which works with Sendmail 8.12
and later. It 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.

There are some caveats you should be aware of before using MIMEDefang.
MIMEDefang potentially alters e-mail messages. This breaks a "gentleman's
agreement" that mail transfer agents do not modify message bodies. This
could cause problems, for example, with encrypted or signed messages.