Earlier today the Exchange CXP team released Update Rollup 4 for Exchange Server 2007 SP3 to the Download Center.
This update contains a number of customer-reported and internally found issues since the release of RU3. For a list of changes included in this rollup, see KBA 2509911 Description of Update Rollup 4 for Exchange Server 2007 Service Pack 3. We'd like to specifically call out the following fixes which are included in this release:
Update Rollup 5 for Exchange Server 2007 Service Pack 3 is currently scheduled to release in August 2011.