Backup Job Delays Due to Microsoft 365 API Throttling

Product version: 11.0

Last modified: 05 June 2025

Problem

Backup or recovery jobs that interact with Microsoft 365 services may fail or experience delays due to API throttling. This happens when too many requests are sent to Microsoft APIs within a short time, triggering automatic throttling responses from Microsoft 365 services.

This is common in large-scale Microsoft 365 backup jobs that:

  • Process a high number of mailbox items or SharePoint documents

  • Run multiple jobs in parallel

  • Use many concurrent tasks per job

Background

Microsoft API throttling is a protective mechanism used by Microsoft to maintain service health and performance. It limits the number of API requests that clients can send over a given time. If too many requests are sent, Microsoft may temporarily reject them or return error codes such as 429 (Too Many Requests) or 503 (Service Unavailable).

For more technical details, refer to:

Solution

Microsoft API throttling is a normal behavior when limits are exceeded. To avoid or reduce throttling, use best practices to minimize request volume and concurrency to keep your backup jobs stable and compliant with Microsoft’s limits.

  1. Decrease the Number of Processed Items. Limit the number of users, sites, or mailboxes selected per job. Splitting large jobs into smaller ones may help reduce the load on the Microsoft API.

  2. Reduce Maximum Concurrent Tasks. Go to the job settings and lower the number of Maximum Concurrent Tasks. This reduces the number of simultaneous API requests.

  3. Wait and Retry Later. Microsoft automatically resets the throttling limit after a cooldown period. If your job is throttled:

    • Pause or delay the job for several minutes to allow throttling limits to reset

    • Reschedule heavy jobs to run during off-peak hours.