This update contains a number of customer-reported and internally found issues since the release of RU1. See 'KB 2509910: Description of Update Rollup 4 for Exchange Server 2010 Service Pack 1' for more details. In particular we would like to specifically call out the following fixes which are included in this release:
Unable to Create a 'Reply With' Rule on Public Folders Even With Owner and Send As Permissions
Generating DSN fails if original mail uses non-support encoding charset.
Outlook 2007 does not deliver "Delayed Delivery" Messages against an Exchange 2010 Server in Online mode with any additional Transport loaded in the Outlook Profile