For those of you preparing to do an inter-org Exchange migration between Exchange 2010 organizations, the MRSProxy service is a required component that facilitates cross-forest mailbox moves. (Note this process is only relevant when one forest is running Exchange 2010 and the other forest has at least a single Exchange 2010 CAS Server). This service is installed by default, but is disabled. As such, it must be enabled when doing this type of migration.
Microsoft published a definitive KB article on how t... read more.