Last weekend our email server hit the database size limit and dismounted the information store, halting all email transport and access.
Originally with Exchange 2003 Standard Edition there was a 16GB limit on the information store. This was one of the factors used to force you to upgrade to the Enterprise Edition which is ridiculously more expensive . When the database hit that size it would dismount causing all your users to start wrapping torches and sharpening pitchforks. They enabled a registry tweak to allow you 1 more GB of space to allow you to mount the store and reclaim some space by deleting old email.
With Service Pack 2 they upped the default limit to 18GB, which is the threshold we hit a few days ago, and allowed an absolute limit of up to 75GB. You can set this limit with another registry tweak.
Of course this couldn’t have happened on a weekday when I was here, it had to happen on a Saturday while I was out to lunch with the family and some friends. Figures…