How big are the messages that are slipping through?
Set the limits to catch those...
If that burries the box, hand the bosses the bill for an upgrade to a C3xx...
You'll need to get off the 150 box anyway, its EOL soon...
Currently I've got a C370 (overkill for the amount of traffic we have now...5200 inbound clean messages today)
Its limits are set to scan everything under 1M, don't scan anything over 25M, and scan for up to 120 seconds.
You could also toss those suspect messages over to Cisco to find out why they aren't getting caught as spam... there may be some tweaking that can be done...