Known Problems With Act!
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Corrupt Act! Database
Removal of Users from an Act! database will corrupt your database and result in fields that cannot be transferred to Outlook/Exchange.  
This is the most common issue we have with Act! when using our product. We've had users that remove any additional users from their database only to see the database become corrupt as a result. Don't do it!  
In general, you'll see strange errors popup and what used to work just fine no longer works.