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I found the technet blog pasted below
1. Stopped the Microsoft Exchange Search Indexer Service
2. Deleted the Catalog folder associated with the mailbox database
3. Started the Microsoft Exchange Search Indexer Service
The folder was at 440MB for a 54GB mailbox database. The folder right now is almost 700MB and growing. I will check back in a few hours to see if the problem is resolved once it rebuilds completely.
If your organization has a need to mass-create AD user accounts, watch this video to see how its done without the need for scripting or other unnecessary complexities.