In troubleshooting a very obscure error in Outlook 2003 (0x800…error unknown), I ran across a very helpful tool called the Inbox Repair Tool (scanpst.exe). This tool can be used to find and fix errors in a .pst or .ost file. Scanpst.exe file is found at C:\Program Files\Common Files\System\MSMAPI\1033\ on a standard Windows XP and Office installation. One of the less obvious things about .pst and .ost files is that they never reclaim unused space. They just keep growing much like Microsoft Access jet databases. If you want to shrink the size of your .pst or .ost file, you must manually compact it. If you use Office 2k3, go to Tools -> Options -> Email Accounts -> View or Change an Existing Account. Then select your Exchange account and click “Change”. Select “More Settings” and on the “Advanced” tab, there is an “Offline Folder File Settings” button. Click that and choose the “Compact Now” button.