The MRSProxy is a component of Microsoft Exchange 2010 that facilitates cross-forest mailbox moves.
Microsoft improved the management of the Exchange 2010 MRSProxy with SP2. There are new cmdlets available to allow for easier management of the MRSProxy component, however, Microsoft did not maintain the TechNet documentation that describes how to do this on systems that do not have SP2 installed. The TechNet article, Start the MRSProxy Service on a Remote Client Access Server, has been updated to only show the SP2 approach.
As a reference, here is how you enable the MRSPRoxy fon Exchange servers prior to SP2.
To enable the MRSProxy service, you need to edit the EWS “web.config” file that is located (Default Install Dir) in “C:\Program Files\Exchange Server\V14\Client Access\exchweb\ews”. The parameter to edit is “IsEnabled” that is located in the section that starts with “<!– Mailbox Replication Proxy Server configuration –>”
Update the web.config line that reads IsEnabled=”false” to IsEnabled=”true”.
I’m not sure if it is required, but I usually restart the Microsoft Exchange Mailbox Replication service after this change is complete. Keep im mind this service is responsible for transaction log shipping, so you may want to shuffle around your active database copies if you are using a DAG or perform this change during a maintenance window.
For more information about cross-forest mailbox moves check out the TechNet article titled, Understanding Move Requests.