Description of Update Rollup 4 for Exchange Server 2007 Service Pack 3
Update Rollup 4 for Exchange Server 2007 Service Pack 3 (SP3) resolves issues that were found in Exchange Server 2007 SP3 since the software was released. This update rollup is highly recommended for all Exchange Server 2007 SP3 customers.

Update Rollup 4 for Exchange Server 2007 Service Pack 3 (KB2509911)
http://www.microsoft.com/download/en/details.aspx?displaylang=en&id=26692

Issues that the update rollup resolves:
http://support.microsoft.com/?kbid=2509911