MIMEDefang is a flexible MIME email scanner designed to protect Windows clients from viruses. However, it can do many other kinds of mail processing, such as replacing parts of messages with URLs, adding boilerplate disclaimers, and so on. It can alter or delete various parts of a MIME message according to a very flexible configuration file. It can also bounce messages with unacceptable attachments.
MIMEDefang works with Sendmail 8.11/8.12s new "Milter" API, which makes it more flexible and efficient than procmail-based approaches.