Event ID : 9690

Issue : Exchange Database size exceeded its Limit, database will be dismounted immediately.

Event Source : MSExchangeIS Mailbox Store

Description : Exchange 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 GB. This database size has exceeded the size limit of GB.
This database will be dismounted immediately.

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=9690&EvtSrc=MSExchangeIS+Mailbox+Store