If you receive an Event ID: 9688 or 9690 Event Source: MSExchangeIS Mailbox Store "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 37 GB. This database size has exceeded the size limit of 18 GB."

To fix the issue, Microsoft implemented a change in Service Pack 2 that supports increasing the database size up to 75GB.

You can configure a logical database size (logical size means the physical size of the .EDB and .STM  files minus the logical free space aka white space in each) limit for each Exchange database by creating a DWORD registry key named “Database Size Limit in GB”. This key should be created under the following location for the mailbox database and public folder database respectively:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ <Servername>\Private-GUID

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\ <Servername>\Public-GUID

GUID is short for Globally Unique Identifier which is a unique 128 bit number generated by Windows or a Windows application, in this case Exchange.

After the Mailbox/Public Folder Store has been configured with a new logical database limit, you will have to dismount/mount the given store (or simply restart the Information Store service) in order for the changes to take effect.