After deleting or moving a large number of mailboxes, you can find out how much free space an offline compaction would reclaim. This is especially important if you're getting close to your database size limit or you're running low on disk space. Shrinking your database also reduces the amount of time it takes to back up or restore your system, and it can improve performance.
To find out how much free space you can regain, look for event 1221 in the Windows NT application event log. (You must be running Exchange 5.5 SP1 or greater.) Exchange Server reports separate 1221 events for the private IS, the public IS, and the Directory store.
The free-space figure reported by event 1221 indicates the minimum amount of space that performing an offline defragmentation would recover.