Since the latest Exchange 2003 IMF update (January 23rd 2007), some users were unable to send to their Exchange inbox the pdf files created with a scan-to-email multifunction printer.
The printer uses SMTP (port 25) to send the file to the Exchange server.
When I disabled the IMF from the SMTP virtual server the problem was solved, but I didn't want to keep it disabled.
So I added a second port in the SMTP virtual server (port 26), enabled IMF on it, and I configured the firewall to forward traffic to this port instead of standard port 25.
This way, internal e-mail goes to port 25 and is not filtered, but messages from external systems are forwarded to port 26 and filtered by IMF.
Edit on 2007/04/24:
Finally I found a "cleaner" solution. Connection filter has an "IP accept" list. If you add the IP of the printer in this list and enable Connection Filtering on the SMTP virtual server, it will bypass IMF.