This update contains a number of customer-reported and internally found issues since the release of RU5. See KB 2608656: Description of Update Rollup 6 for Exchange Server 2007 Service Pack 3' for more details.
We would like to specifically call out the following fixes which are included in this release:
For DST Changes: http://www.microsoft.com/time.
Note for Forefront Protection for Exchange users For those of you running Forefront Protection for Exchange, be sure you perform these important steps from the command line in the Forefront directory before and after this rollup's installation process. Without these steps, Exchange services for Information Store and Transport will not start after you apply this update. Before installing the update, disable ForeFront by using this command: fscutility /disable. After installing the update, re-enable ForeFront by running fscutility /enable.
Read the Microsoft Exchange team blog at http://blogs.technet.com/b/exchange/archive/2012/01/26/released-update-rollup-3-for-exchange-2007-service-pack-3.aspx
Read my favorites blogs:
Designing a backup less Exchange 2010 Architecture
Step by step guide for upgrading Active Directory from Microsoft Windows 2003 to Microsoft Windows Server 2008
Microsoft Exchange 2010 CAS Array – Steps and Recommendations
Appear Offline in Microsoft Office Communicator Server 2007
Microsoft Exchange 2010 Test cases
Microsoft Exchange Server 2010 Disaster Recovery