The Microsoft Exchange Mailbox Replication Service (MRS) is a feature that handles mailbox import, export, migration and restoration requests for Exchange Server. MRS is the principal mechanism used to move a mailbox from a source database to a destination database. Mailbox moves can take place between databases on the same server, and between databases on different servers, different domains, different Active Directory (AD) sites, or different AD forests.
Administrators can use the Exchange Management Shell and the Exchange Admin Center to configure and execute mailbox moves. These tools handle moves for a single mailbox but can also perform multiple simultaneous mailbox moves the administrator executes either manually or through the PowerShell New-MigrationBatch or New-MoveRequest cmdlets.
Asynchronous feature prevents disruptions
Microsoft Exchange Mailbox Replication Service moves mailboxes asynchronously to keep mailboxes online for users during migrations. Exchange generates a content index once the move starts and allows users to search mailboxes once the move completes.
Exchange Server 2013 and Exchange Server 2016 use the same batch move architecture which features large batch support, email notifications with reporting during moves, automatic retry and move prioritizations, movement of personal and archive mailboxes as a group or individually, and recurring resynchronization to incorporate any mailbox changes during the move.
MRS supports throttling while asynchronous moves execute in the background. Throttling prevents excessive demands for compute or network resources to maintain performance of regular Exchange operations and other applications on the network.
Administrators configure MRS through the MSExchangeMailboxReplication.exe.config file located on all client access servers. Administrators can adjust key parameters, such as MaxRetries, RetryDelay, MaxActiveMovesPerSourceServer, MaxActiveMovesPerTargetServer and MaxTotalMovesPerMRS. Each MRS instance, mailbox database or mailbox server can have its own configuration settings.
MRS Proxy needed for certain mailbox moves
Although Microsoft Exchange Mailbox Replication Service supports local migrations, the MRS Proxy is required for cross-forest mailbox moves and remote moves between on-premises Exchange deployments and Exchange Online services.
Administrators must enable the MRS Proxy in client access servers to facilitate cross-forest and remote move requests. For example, the MRS Proxy must be enabled on source client access servers when moving mailboxes to another forest from the destination -- known as a pull move. The MRS Proxy must be enabled on the destination client access servers when moving mailboxes from the source -- called a push move.
Any remote moves between on-premises Exchange and Exchange Online require MRS Proxy running on the on-premises client access servers.
Exchange mailbox movement scenarios
There are several scenarios where it might be necessary to move a mailbox. As part of the Exchange Server upgrade process, the administrator might transfer mailboxes from the older version to the new version of Exchange. Administrators might move mailboxes to balance database sizes to achieve more uniform database performance or place a user's mailbox in a location closer to the user's physical location to reduce potential latency issues.
Administrators also shift a mailbox to diagnose and correct email problems or to address mailbox corruption. To isolate Exchange administration from Windows operating system administration, the IT department can transfer mailboxes into separate AD forests.