You should plan for your anti-spam scanner malfunctioning someday
Yesterday I mentioned that the commercial anti-spam and anti-virus system we use ran into a bug where it hung up on some incoming emails. One reaction to this is to point and laugh; silly us for using a commercial anti-spam system, we probably got what we deserved here. I think that this attitude is a mistake.
The reality is that all modern anti-spam and anti-virus systems are going to have bugs. It's basically inherent in the nature of the beast. These systems are trying to do a bunch of relatively sophisticated analysis on relatively complicated binary formats, like ZIP files, PDFs, and various sorts of executables; it would be truly surprising if all of the code involved in doing this was completely bug free, and every so often the bugs are going to have sufficiently bad consequences to cause explosions.
(It doesn't even need to be a bug as such. For example, many regular expression engines have pathological behavior when exposed to a combination of certain inputs and certain regular expressions. This is not a code bug since the RE engine is working as designed, but the consequences are similar.)
What this means is that you probably want to think ahead about what you'll do if your scanner system starts malfunctioning at the level of either hanging or crashing when it processes a particular email message. The first step is to think about what might happen with your overall system and what it would look like to your monitoring. What are danger signs that mean something isn't going right in your mail scanning?
Once you've considered the symptoms, you can think about pre-building some mail system features to let you deal with the problem. Two obvious things to consider are documented ways of completely disabling your mail scanner and forcing specific problem messages to bypass the mail scanner. Having somewhat gone through this exercise myself (more than once by now), I can assure you that developing mailer configuration changes on the fly as your mail system is locking up is what they call 'not entirely fun'. It's much better to have this sort of stuff ready to go in advance even if you never turn out to need it.
(Building stuff on the fly to solve your urgent problem can be exciting even as it's nerve-wracking, but heroism is not the right answer.)
At this point you may also want to think about policy issues. If the mail scanner is breaking, do you have permission to get much more aggressive with things like IP blocks in order to prevent dangerous messages from getting in, or is broadly accepting email important enough to your organization to live with the added risks of less or no mail scanning? There's no single right answer here and maybe the final decisions will only be made on the spot, but you and your organization can at least start to consider this now.