By default, both on-premise Exchange and Office 365 use throttling policies to help ensure that a single user does not use up their fair share of Exchange server resources. Under normal conditions, this ensures that the Exchange server remains healthy and available to all end-users. However, during a migration the default Exchange throttling policy will cause significant migration performance impacts. We would suggest creating a new "migration" throttling policy in order to achieve the fastest, error-free migration possible.
The following migration throttling policy is appropriate for migration to and/or from on-premise Exchange 2013 and Exchange 2016 servers.
1.) Open an Exchange Management Shell.
2.) Run the following cmdlets:
Set-ThrottlingPolicy MigPolicy -RCAMaxConcurrency Unlimited -EWSMaxConcurrency Unlimited -EWSMaxSubscriptions Unlimited -CPAMaxConcurrency Unlimited -EwsCutoffBalance Unlimited -EwsMaxBurst Unlimited -EwsRechargeRate Unlimited
Set-Mailbox MigAdmin -ThrottlingPolicy MigPolicy
Where 'MigAdmin' is the name of your migration admin account.
3.) Restart the Exchange Transport Service to activate the the new throttling policy settings.
In order to minimize Office 365 throttling impact to migration and to raise the overall migration throughput, we highly recommend upgrading your Office 365 tenant throttling policies. Please contact Microsoft support with the request to raise the limits for the following throttling parameters to 'Unlimited':EwsMaxBurstEwsRechargeRateEwsCutoffBalance
This the throttling limits should be raised for each Office 365 tenant in the migration project. The upgrade can be done for the time of your migration only.
Note: Microsoft typically implements throttling policies with an automated expiry in a set period of time. The new migration throttling policy expiration should be noted in case the migration project takes longer than expected. If the migration throttling policy expires before the migration project is complete, then you will need to re-engage Microsoft to extend the migration throttling policy.