Exchange Database Dismounting each night

Monday morning and no company e-mail!!
Server performance report showed critical error from MSExchangeIS Mailbox Store that database limits exceeded (22GB).
6 users were taking 60% of the available 18GB!
After archiving, spaced now used 14GB.
But Tuesday and Wednesday morning we still have the same problem and reported critical error about exceeding limits. (Mailbox starting size still reading 22GB, Ending size reading 14GB)
By restarting Exchange Server Services (Information Store & System Attendant) we get e-mail for the rest of the day. but when the maintenance routines run at 5:00 am the critical error (Event ID: 9690) reports and the database is dismounted again.
Exchange is not recognising the reduction in Mailbox size.
Can anybody help me with this problem?

Thanks in advance

Neil