If you receive the error 429 too many requests during the backup of your 365 environment using Veeam backup for Microsoft 365, you may suffer a Microsoft throttling issue.
Throttling limits the number of concurrent calls to a service to prevent overuse of resources. This may occur if you are performing for example full backups or a large volume of writes and reads tasks.
If an overwhelming use of resources is detected, Microsoft starts throttling to keep optimal performance of the service.
Error 429 too many requests
Looking at the Veeam Backup for Microsoft 365 console, the backup job is trying to connect the organization but it takes ages and it doesn't move forward.
Analyzing the backup job log, it reports the following error:
- error 429 too many requests
Microsoft throttling
When you receive such an error, it means that throttling is occurring in your system and it is due when a tenant receives a large number of requests from a specific application (Veeam Backup for example) and the threshold is exceeded.
When the threshold is exceeded, Microsoft limits the requests from the specific application for a period of time. When there are too many requests, Microsoft Graph returns HTTP status code 429 and the requests fail. This is the reason for the error 429 too many requests in Veeam Backup for Microsoft 365.
Change the throttling threshold
The error 429 too many requests may occur especially during the initial full backup of your 365 tenant (Mailboxes, SharePoint, Teams, OneDrive) when a large number of requests are received from the backup application.
To allow a bigger amount of requests for a limited period of time, the change of the throttling settings is the solution you can adopt.
Changing throttling settings
Login to your Microsoft 365 admin center and click on the ? (help) symbol.
Type ews and select EWS Throttling Policies Increase to access the correct section.
Click Run Tests to check if the service is throttled.
Tests are being performed.
If the tests confirm the service is throttled, you can change the Throttling Policy to allow the task (full backup for example) to complete.
Specify the desired Duration (Days) by selecting 30, 60 or 90 as values from the drop-down menu and thick the acknowledge warning. Click Update to change the policy settings.
The system requires about 15 minutes to complete the update.
Handle throttling
If the backup application is affected by the error 429 too many requests, you may adopt the following recommendations to handle throttling:
- Reduce the number of operations per request.
- Reduce the frequency of calls.
- Avoid immediate retries.
Running the backup task once again, the application now runs properly.
After completing the initial full backup, subsequent incremental backups should not face the issue.
