Microsoft introduced Scanost with Outlook 2007 specifically to deal with OST files. Earlier versions of Outlook include Scanpst.exe that can repair OSTs and PSTs, and I can only conclude that the Scanost program is smarter at dealing with internal OST structures than Scanpst. When Scanost is finished processing, it creates a message in your Deleted Items folder to report its activity. Figure 3 shows a sample message.
If an OST still doesn't perform well after compacting and scanning, the last thing you can try to restore an OST to good health is to rebuild it from scratch. Think of this as the equivalent to running Eseutil to rebuild an Exchange database. You have to exit Outlook and delete the OST—although people who are more cautious might opt to rename it instead, just in case—and then restart Outlook to force the client to recreate the OST.
Just like you wouldn't recommend that Exchange administrators rebuild databases regularly—a task not necessary with Exchange 2003 and later, even if the mythical need persists in the imaginations of many—rebuilding OSTs isn't something to recommend users do regularly. Recreating an OST can take a few hours depending on its size, number of items it contains, and the speed of the network connection to the Exchange server. You definitely don't want to recreate an OST over a slow network link or when the server is under heavy load because these factors slow processing considerably.
The Importance of OST Speed
A rebuilt OST isn't a guarantee of snappy performance; the inherent flaws of the OST structure when coupled to a large mailbox can still condemn you to slowness. That's why it was so important for Microsoft to invest some engineering effort to help OSTs cope with large mailboxes. Microsoft has known for some time that large OSTs perform like slow pigs, and it's surprising that they didn't address the problem in Outlook 2007. The growing size of mailboxes and the increasing number of mailboxes connected to Exchange 2007 and Exchange 2003 servers made the problem more evident.
In addition, Microsoft needs to make it easier for users to have even larger mailboxes in the future so that it can compete with Google and other online vendors that offer free mailboxes in the 5GB to 10GB range. Some organizations already run Exchange servers with very large mailboxes; a 50GB mailbox isn't uncommon for a senior corporate executive, especially those who are targets of legal discovery actions. The slowness of the OST can make using large mailboxes a real pain even if you're using the fastest, most up-to-date hardware.
The good news is that the fixes now available for Outlook 2007 SP1 and in Outlook 2007 SP2 deliver much better performance and responsiveness for OSTs connected to large mailboxes. It’s hard to measure just how good the improvement is because no tools are available for this purpose. My nonscientific tests show that the beta version of Outlook 2007 SP2 spends less time causing the hard disk light to come on and provides a much more responsive performance with mailboxes of up to 10GB—in other words, you definitely experience fewer "stutters" when Outlook opens a large folder or changes views. I didn't attempt to test a mailbox larger than 10GB. I anticipate that the final SP2 code will deliver even better performance after Microsoft removes the debugging code that it usually includes in beta software.
Relieving a Shut Down Problem
Along with speeding OST performance, Outlook 2007 SP2 changes how the program shuts itself down. The details are explained in "Application Shutdown Changes in Outlook 2007 SP2," but basically the problem was that Outlook took too long to shut down after a user requested the application to close. Users would sometimes become frustrated that Outlook didn't shut down as promptly as they expected, which led them to terminate the process using Windows Task Manager.
Forcing Outlook to quit with Task Manager certainly is effective. However, this process can cause Outlook to fail to write cached data to disk properly and so corrupt the OST. The next time Outlook starts, it detects the corruption and fixes it, but while Outlook is scanning for problems and fixing whatever it finds, it's slower to respond to user requests and therefore creates the impression of being a performance hog. The change to force snappier shutdowns makes users happy and stops itchy users wanting to kill processes, so all in all it's a good thing.
Upgrade for Better Performance
It would be nice if every change that Microsoft makes to Outlook had such an obvious and immediately positive effect as the company managed to do in these updates for OST performance. Deploying a new service pack or hotfix for any software can be a painful and costly affair, especially if you lack the ability to distribute the new software and apply it automatically to all the PCs in your organization. It's not always obvious that the costs of such an exercise will result in any measurable benefit.
However, if you have the opportunity to deploy the February 2009 Cumulative Update or Outlook 2007 SP2 in the near future, the enhanced performance will delight any user whose mailbox is larger than 1GB. Given our ability to be human packrats and grow mailboxes to sizes that we never contemplated a few short years ago, better performance and support for large mailboxes could be just the reason you need to justify the upgrade.