MIMEDefang is an e-mail filter program which works with Sendmail 8.12 and later, or Postfix. 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 users 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.