For those of you that don’t already have this information handy:
Exchange Standard 2000 & 2003 (pre-SP2) has a hard mailbox limit of 16GB and the store will dismount when this limit is reached. There is a registry edit detailed here (MS KB 813051) which is used to permit you to mount the store and perform maintenance (purging deleted items, mailbox cleanup, dumping data to PST, archiving and deleting terminated employees, etc) – and then of course performing an off-line defrag to reclaim that space. Once the store size is under 16GB, you must then undo the registry change so your limit is back to 16GB (if you leave it at 17GB, you could paint yourself in a corner the next time the store grows). After that, depending on the client (since I’m not aware of an Apex standard) you should look at implementing one or more of the following: mailbox limits, mailbox maintenance (purging deleted items etc), implementing e-mail archiving to PST files, etc.
Exchange Standard 2003 (SP-2 and later) comes out of the box with a 16GB limit on the information stores, but you can change this to up to 75GB which is detailed here (MS KB 912375). I would recommend you choose a size smaller than 75GB, based on two factors: (1) you should set a store size smaller than your available physical storage; (2) you want it smaller than 75GB so if it reaches that limit, you’ve got wiggle room to increase it and perform maintenance to decrease the stored data.