Microsoft rate-limiting error when fetching attachments via Nylas

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1

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Microsoft rate-limiting error when fetching attachments via Nylas

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Hi everyone,

Sometimes I keep getting this error from Nylas when trying to fetch an email attachment:

Failed to get attachment: Microsoft rate-limited on application level. Please read provider error message in this response for more details. Learn more: https://learn.microsoft.com/en-us/graph/throttling-limits#limits-per-mailbox

I understand that Nylas may send multiple API requests to the provider to fulfill a single client request, which is reflected in the nylas-provider-request-count header.
However, in my case, this value is 1.

I also checked my system traces — the traffic is around 2–3 requests per second, which should be far below Microsoft’s rate limits.

Has anyone encountered this issue before or knows what could be causing it?
Should I contact Nylas support directly, or is there something I can do on my end to avoid this?

Thanks in advance!

2

Hello,

Thanks for contacting Nylas Support. We’d be happy to help.

Understanding the Issue
You’re encountering Microsoft’s concurrent request limit rather than a total volume limit. Even though nylas-provider-request-count: 1 shows that each Nylas request makes only one Microsoft Graph API call, the rate limiting is triggered by concurrent requests to the same mailbox.

Why This Happens
Microsoft Graph enforces a maximum of 4 concurrent requests per mailbox. When you make 2–3 requests per second, if they overlap in execution time, Microsoft counts them as concurrent requests. Once you exceed 4 simultaneous requests to the same mailbox, you’ll receive the ApplicationThrottled error—even if your overall request rate is low.

The Nylas-Provider-Request-Count header only reflects the number of provider API calls per individual Nylas request, not the total concurrent load on the mailbox. Additionally, if you have other operations running (such as sync, calendar operations, or message fetching), these also consume the same concurrency quota.

Recommended Solutions

  1. Serialize Your Requests
    Ensure attachment downloads happen sequentially rather than in parallel

  2. Implement Retry Logic with Backoff
    Nylas returns a Retry-After header when Microsoft rate-limits a request. Use this to implement proper retry logic:

  3. Limit Concurrent Requests Per Mailbox
    Implement a concurrency limiter to ensure you never exceed 3 concurrent requests per mailbox (leaving a buffer):

  4. Check for Other Concurrent Operations
    Verify whether other operations are running for the same grant:

Additional Resources

Next Steps
Please try implementing request serialization or concurrency limiting as described above. If you continue to experience issues after making these changes, please let us know.

Feel free to reach out if you have any questions.

Many thanks,
Samuel R.
Support Engineer, Nylas