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:
2519359Unable to Create a 'Reply With' Rule on Public Folders Even With Owner and Send As Permissions
2394554Generating DSN fails if original mail uses non-support encoding charset.
2490134 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
Some of the above KnowledgeBase articles are not replicated/live at the time of writing this post. Please check back later in the day if you can't reach them.
Update Rollup 5 for Exchange Server 2010 Service Pack 1 is currently scheduled to release in August 2011.
Note for Exchange 2010 Customers using the Arabic and Hebrew language version: We introduced two new languages with the release of Service Pack 1, Arabic and Hebrew. At present we are working through the process of modifying our installers to incorporate these two languages. Customers running either of the two language versions affected are advised to download and install the English language version of the rollup which contains all of the same fixes.
Note for Forefront users: For those of you running Forefront Security 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.