Article ID: 2654376 - View products that this article applies to.
In a hybrid deployment of Microsoft Exchange Online in Microsoft Office 365 and on-premises Microsoft Exchange Server, you receive the following error message when you move mailboxes from your on-premises environment to Office 365:
Mailbox Move to the cloud fail with error: Transient error CommunicationErrorTransientException has occurred. The system will retry
This issue may occur if the on-premises Microsoft Forefront Threat Management Gateway (TMG) server blocks HTTP requests from servers in the Office 365 environment. This occurs because of the flood mitigation feature in the Forefront TMG. This mechanism helps lessen flood attacks.
To confirm that this is the issue that you are experiencing, examine the alerts that are displayed in the Forefront TMG management console. To do this, follow these steps:
To resolve this issue, configure flood mitigation settings in Forefront TMG to increase the custom limit for the maximum number of HTTP requests per minute. To do this, follow these steps:
For more information about this issue, visit the following Office 365 Community website:
Office 365 Move Mailbox fails with transient exception
For more information about flood mitigation in the Forefront TMG, visit the following Microsoft TechNet website:
Overview of flood mitigation
Still need help? Go to the Office 365 Community