Event ID : 9689

Issue : Exchange Database size reached its Limit, database will dismount anytime.

Event Source : MSExchangeIS Mailbox 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 size of this database is not reduced to less than the maximum size limit, this database will be dismounted the next time a database size check is performed.

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.

Work Around: How to increase Exchange database size for 1 GB Extension.
Add the Temporary DB Size Limit Extension registry entry to the computer that is running Exchange 2003 Standard Edition or Exchange 2000 Standard Edition:
a. Open Registry Editor.
To do this, click Start, click Run, type regedit in the Open box, and then click OK.
b. Locate the following key in the registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\\Private-
c. On the Edit menu, point to New, and then click DWORD Value.
d. Type Temporary DB Size Limit Extension, and then press ENTER.
e. Double-click Temporary DB Size Limit Extension.
f. Type 1 in the Value data box, click Decimal in the Base box, and then click OK.
g. Close Registry Editor.

Ref.Link :
http://www.microsoft.com/technet/support/ee/transform.aspx?ProdName=Exchange&ProdVer=6.5.7638.0&EvtID=9689&EvtSrc=MSExchangeIS+Mailbox+Store&LCID=1033