MSExchangeIS Mailbox Store
Exchange store 'First Storage Group\Mailbox Store (SERVER)': The logical size of this database (the logical size equals the physical size of the .edb file and the .stm file minus the logical free space in each) is 16 GB. This database size is approaching the size limit of 18 GB.
If the logical database size exceeds the maximum size limit, it will be dismounted on a regular basis.
For more information, click http://www.microsoft.com/contentredirect.asp.
The Exchange server has SP2 which allows you to increase the limit of the data store. However, it doesn't do this by default. Registry keys must be added. The needed keys are listed in the included link.