We recently asked our users to delete unnecessary mail items from their Outlook clients. Now that we've cleaned out the deleted items, we want to recover the disk space the mail items were using. Although we ran a complete offline backup of the server, then ran Exchange Performance Optimizer, the available space appears to be unchanged. How can we reclaim this space?
The only way to reclaim unused space, called white space, in an Exchange Server database is to defragment the database. The Information Store (IS) performs a daily online defragmentation—look for event ID 1221 to tell you how much white space is available. If the amount of white space is significant, you can take the database offline and defragment it manually by using Eseutil. Remember, something might go wrong while you're defragmenting or remounting the database, so before you defragment the database, always make and verify a complete online backup. I recommend that you don't perform the offline defragmentation unless you really need the white space.