"Mailbox Replication Proxy Service can't process this request because it has reached the maximum number of active MRS connections allowed" error during a hybrid mailbox move

Applies to: Exchange OnlineExchange Server 2013 EnterpriseExchange Server 2013 Standard Edition More

Note The Hybrid Configuration wizard that's included in the Exchange Management Console in Microsoft Exchange Server 2010 is no longer supported. Therefore, you should no longer use the old Hybrid Configuration wizard. Instead, use the Office 365 Hybrid Configuration wizard that's available at http://aka.ms/HybridWizard. For more information, see Office 365 Hybrid Configuration wizard for Exchange 2010.


When you move mailboxes between the on-premises Microsoft Exchange Server environment and Microsoft Exchange Online in a hybrid deployment, you notice that the migrations are stalled or seem to be stopped. When you view the move logs, you see the following error message:
MRSProxyConnectionLimitReachedTransientException: The Mailbox Replication Proxy Service can't process this request because it has reached the maximum number of active MRS connections allowed. Current connections: 100, Connections limit: 100.


This issue occurs if the number of moves that use a migration endpoint exceed the default limit of 100 concurrent migrations.


To fix this issue, increase the number of maximum concurrent connections for the Mailbox Replication Proxy (MRSProxy) service.

Warning Increasing these values can affect resources and performance on the on-premises servers. Adjust these settings with caution.

In Exchange Server 2013

  1. Open the MsExchangeMailboxReplication.exe.config file that's located at C:\Program Files\Microsoft\Exchange Server\V15\Bin.
  2. Change the value for MaxMRSConnections from 100 to 320:
    <MRSProxyConfiguration MaxMRSConnections="320" check
Note You have to update this setting every time that you install a cumulative update for Exchange Server.

In Exchange Server 2010

On each Client Access server that has MRSProxy enabled, increase the number of concurrent connections to a value that's slightly larger than the default value for Exchange Online. For example, change the value to 320 concurrent connections.

To do this, open the Exchange Management Shell, and then run the following command:
Set-WebServicesVIrtualDirectory –server <ServerName> –MRSProxyMaxConnections 320 


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