You need to carry out a Microsoft 365 tenant to tenant migration and need to maximize available throttling for Exchange Web Services (EWS).
Request from Microsoft 365 technical support to raise the limits for the EWS throttling policy. The throttling limits should be raised for all involved Microsoft 365 tenants in a migration project. Microsoft implements throttling policies with an expiry time. The migration throttling policy expiration must be noted and if it needs to be extended, then another Microsoft 365 technical support ticket will need to be created with Microsoft.
Inside the Microsoft 365 tenant admin console, click on Support –> New Service Request.
Provide the request details while the “Search” option is chosen.
If you are using third party tool for the migration, EWS throttling should be good to be increased. When you go to “need help” button, you can search for “Increase EWS Throttling Policy” and click enter. First they make you check to see if throttling is affecting the tenant by clicking on “Run” to run the diagnostic test. After you run the Microsoft automated test, you will be able to determine your tenant’s EWS throttling policy.
Then if the result of the test indicates that your tenant is throttled, you can click “Update Settings” to Increase the limit with the desired expiry time.
Microsoft asks you to please run the automated test which will allow you to modify the EWS throttling policy for the duration of your migration. Note that even after this is run, EWS imports will still be limited to 150mb per 5 minutes per mailbox; to achieve higher migration throughput speeds, please migrate more users concurrently. Please note that EWS throttling policy changes have no effect on the following migration types (using Microsoft tools): Hybrid, Cutover/Staged (RPC/HTTP), IMAP, G Suite, Public Folder or PST Import Service.
Microsoft cannot provide more precise information about the throttling policies and what exactly happens in the backend, but that this diagnostic test is specifically designed for helping increase performance for third party migrations.
Generally Microsoft clients are able to do the above throttling policy change via self service in the Microsoft 365 admin portal. However for some reason, it is not working in admin center for some clients. In this case, a Microsoft 365 technical support ticket should be created to escalate the case and carry out the request from the Microsoft side.
Some background reading on EWS throttling policies can be found at: https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/ews-throttling-in-exchange.