MRS (Mailbox Replication Service) is a service in Microsoft Exchange Server that is responsible for replicating mailbox data between mailbox servers. The MRS Proxy service is a component of MRS allowing mailbox moves to be performed between servers that are in separate Active Directory sites.

This can be useful in scenarios where mailbox data needs to be moved between servers in different locations. If the MRS Proxy service is not enabled on a Client Access server, mailbox moves to or from that server may not be possible.

To enable the MRS Proxy service, you will need to use the Exchange Management Shell to run the following command:

Get-WebServicesVirtualDirectory | Set-WebServicesVirtualDirectory -MRSProxyEnabled $true