Update Rollup 2 for Exchange Server 2010 SP2 contains a number of customer-reported and internally found issues since the release of SP2 RU1. See KB2661854: Description of Update Rollup 2 for Exchange Server 2010 Service Pack 2 for more details.
One of the key issues addressed by this rollup is that it fixes the issue in KB2696913 : You cannot log on to Outlook Web App when a proxy is set up in an Exchange Server 2010 environment.
Update Rollup 7 for Exchange Server 2007 SP3 contains a number of customer-reported and internally found issues since the release of SP3 RU6. See KB2655203: Description of Update Rollup 7 for Exchange Server 2007 Service Pack 3 for more details.
For Exchange Server and Update Rollups Builds Numbers kindly visit http://social.technet.microsoft.com/wiki/contents/articles/240.exchange-server-and-update-rollups-builds-numbers.aspx
This Script can be used to find Exchange 2007 and Exchange 2010 build numbers in your environment.
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.