Event ID : 9688

Issue : Exchange database reaching its license limit.

Event Source : MSExchangeIS Mailbox Store , MSExchangeIS Public Store

Description : Exchange store '<storage group name>': 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 <number> GB. This database size is approaching the size limit of <number> GB.
If the logical database size exceeds the maximum size limit, it will be dismounted on a regular basis.

Action Required : To reduce the size of the database, consider doing the following:
1) Delete mailbox or public folder data that you do not need. For example, ask your users to delete items in their mailboxes or delete public folder data that is not needed.
2) Move mailboxes or public folders from the database to a different database.
3) Take offiline Backup.
4) perform an offline defragmentation with the Eseutil.exe utility (the eseutil /d command) against the .edb database file.

Ref.Link : http://www.microsoft.com/technet/support/ee/transform.aspx?ProdName=Exchange&ProdVer=6.5.7638.0&EvtID=9688&EvtSrc=MSExchangeIS+Mailbox+Store&LCID=1033