Applies to Exchange 2003 

  So one of the things that can go wrong with Exchange is that it can run out of something called Version Store.  Version store is an in-memory list of changes made to the database.  Nagesh Mahadev has an awesome post about Version Store on our msexchangeteam.com blog, posted here.  To borrow his summary:  In simple terms, the Version Store is where transactions are held in memory until they can be written to disk.

  So version store running out of memory can be caused by either a long running transaction.  This is pretty self explanatory.  Say your anti-virus product wants to scan something in VSAPI and locks it and then goes to lunch.  Your version store will consume more and more memory until it runs out because it's trying to work around this long running transaction, keeping track of all the rollbacks and whatnot.

  The other problem is with I/O.  Since we're holding transactions in memory until they can be written to disk, if something prevents us from writing to disk, we can hit version store problems.  Sometimes this type of problem can be precipitated by 9791 event log entries in the application event log.  If this happens, get ready to do some adplus store dumps when version buckets allocated hits 70%.

What to do to prevent or mitigate this risk?

1. Consider increasing transaction log buffers, especially if you are seeing transaction log stalls in your environment.  The logic here is that if store can't commit transactions to the log files fast enough, it can cause version store to back up.  By default the number of buffers is 500, you can increase this to 9000.  This will prevent a single database from needing to write a bunch of TLs at once and backing up version store.  I highly recommend using the EXBPA for governance on this, details on the rule for setting this, etc can be found here.
2. Watch your PTE resources and treat accordingly.  I've seen customers run low on free PTEs and run into version store problems because they don't have the capacity to perform IO operations as fast as the database would like.
3. Make sure your online maintenance is completing frequently, at least once a week on each database.  Part of online maintenance is defragmenting your database.  On a highly fragmented database(s) version store has to keep track unoptimized links and tables and dealing with records that are not on the fewest number of pages possible, in essence bloating version store size with each transaction.  For indepth information on Exchange Store Maintenance, go here.
4. Keep your message size limits down.  Going hand in hand with this is preventing older Outlook clients from accessing your server.  Old clients (Older than Outlook 2003 SP2 in cached mode, any version of Outlook 2003 and higher for online mode) ignore your message size limits for submitting messages, so older clients could attach a 100 meg file and submit and store would have to deal with it even though it's over the size limit.  This should give you the gist of what I'm talking about here.

Hope this helps with your environment.