EWS (Exchange Web Services) throttling can slow down archive and group mailbox migrations.
If your migration is running slower than expected, you can submit a service request to Microsoft to temporarily lift your EWS throttling limits.
Note: EWS throttling applies only to migrations that include archive mailboxes and group mailboxes.
Before you start
You need global admin access to the Microsoft 365 admin center.
Request the increase
Go to admin.microsoft.com and sign in with a global admin account.
In the left navigation, select Support.
Select New service request.
In the search bar, type throttling.
Select Increase EWS Throttling Policy from the results.
Complete the request form.
Submit the request.
Microsoft grants temporary increases for 30, 60, or 90-day windows. The submission process takes about 15 minutes, and approval is at Microsoft's discretion.
Note: If you're running a cross-tenant migration, you need to submit the request on both the source and destination tenants.