Error: "ExchangeServerBusyError - 401 ErrorServerBusy"

Microsoft Exchange backups fail with the error message, "ExchangeServerBusyError (401 ErrorServerBusy - The server cannot service this request right now. Try again later)."


  • Datto SaaS Protection 2.0 for Microsoft 365


This error may appear in the Failure Reason column of the SaaS Status page when the Exchange account has reached its API rate limit and Microsoft has applied throttling to it.


To maintain performance and service availability, throttling for Exchange Web Services (EWS) is active for all users by default. If the issue is limited to one or two backups, the Exchange server may be temporarily busy and the issue will likely resolve itself. However, if the problem is of a larger scale or the backup failures persist, you can temporarily disable rate limiting via the Microsoft 365 admin console. To do so, perform the following steps:

  1. Log in to the Office 365 Admin page.

  2. In the left navigation pane, locate and expand the Support category.

  3. Click the Help & support option.

  4. The How can we help? modal will populate. Search for Increase EWS Throttling Policy. Then, in the search results, locate the Run diagnostics option and click Run Tests.

  5. Diagnostic tests will run. At their conclusion, you'll see a dialog box indicating that the Exchange Web Services are throttled. In the Duration (Days) drop-down, select the number of days for which you'd like to temporarily modify your throttling policy.

  6. Select the acknowledgment check box. Then, click Update Settings.

  7. You'll see a message stating that an update to your EWS throttling policy is now in progress.

  8. After approximately 15 minutes, repeat steps 1 - 6 above to confirm that your services are no longer throttled. You should see the confirmation shown in the following screenshot.

  9. Attempt your Exchange backups again. If the problem persists, contact Microsoft support for assistance.