Issue:

With a Hybrid deployment (Exchange 2010 and Office 365), you will be using MRS/MRSProxy to perform the mailbox moves between premises. This operation can potentially fail (could be intermittent) when you traverse a TMG server. The reason for this is a defense mechanism built into TMG called Flood Mitigation. Read more about Flood Mitigation here http://technet.microsoft.com/en-us/library/cc995196.aspx . 

If you were to run into this issue you would see an error while migrating the mailboxes similar to the following:  “Mailbox Move to the cloud fail with error: Transient error CommunicationErrorTransientException has occurred. The system will retry”.

To verify this is your issue you could review the Alerts section in the TMG management interface. To find the Alerts section you would open the Microsoft ForeFront TMG management interface. Then in the tree on the left you would go to Monitoring , which will expose the Alerts tab in the middle pane. In there you can find the alerts which says the following:

Mitigation for the issue:

There is a default threshold setting for Flood Mitigation within the TMG interface, which can be found in the Microsoft ForeFront TMG management interface. Once you are in there just select the Intrusion Prevention System option from the tree on the left. This will bring up the following option:

You can see there is an option in the Behavioral Intrusion Detection tab called Configure Flood Mitigation Settings, which when selected will so the following dialog box.

You should then go to the IP Exceptions tab and enter the IP addresses that the Office 365 environment will use to connect to your server during the move operation. The list can be found here http://help.outlook.com/en-us/140/gg263350.aspx

Then from the Flood Mitigation tab you will see the button for Maximum HTTP Requests per minute per IP address you will want to raise the limitation ONLY for the IP addresses in the exceptions list as shown. Please note that 6,000 may not be enough depending on the number of mailboxes being moved, If the error returns you can simply raise the limits to a higher number.

When the settings are in place and applied you should then be able to move the mailboxes without issue.

Timothy Heeney (MSFT)