A number of third party migration solutions use Exchange Web Services (EWS) for migrations to Microsoft 365. This protocol allows these mailbox migration tools to inject migrated items into the target mailboxes in Exchange Online using delegation or impersonation.Throttling policies help ensure server reliability and uptime by limiting the amount of server resources that a single user or application can consume in Exchange Online. When high load factors are detected that degrade the performance of these resources, EWS connections are dynamically throttled based on the amount that each caller has contributed to this high load condition.
The result is that the third-party migration tool that uses EWS may be impacted by the EWS throttling policy. Migrations may slow or stop altogether.
See EWS throttling in Exchange for a full description of EWS throttling and limits.
Note: It's important to call out that EWS throttling policies do not affect Exchange hybrid migrations. Hybrid migrations use MRS not EWS. Do not use the steps in this article to try resolving slow hybrid migrations.
