Your Mailserver does not have to send nonsense bounces anymore.
The software is a sendmail milter, written in python. It is plugin - enabled and tests the mail content after the remote server finished the SMTP "DATA" step. At this place wumi is able to reject a mail. The server has seen it, but refuses to deliver it. Thus we do not need to send bounces (to faked adresses!) nor do out users have to deal with nonsense mails (like "our virus scanner removed the attachment" or the like).
We at wu-wien.ac.at plan to include a bayes-based plugin in this software, so we will be able to reject spam *during the initial smtp connection*.
The software is in production use at wu-wien.ac.at, you can test the behavior by e.g. sending an ".exe" attachment to firstname.lastname@example.org. Our mailserver will block this, and you will see how the software works.