Azure communication services error 429 not resolving

Question

I use Azure email service with an Azure managed domain, for which I understand the rate limit would be 10 emails per hour. I exceeded this limit several hours ago and got the error 429 (too many requests). Despite waiting for several hours I still cannot send emails, is there a cool down period? Or is there another rate limit I may be exceeding, perhaps daily?

Answer 1

Azure Communication Services email imposes strict per-minute and per-hour rate limits, and when these are exceeded, requests are rejected with HTTP 429 (Too Many Requests). For Azure-managed domains, the documented limits are:

• 5 emails per subscription per minute
• 10 emails per subscription per 60 minutes

When these limits are exceeded, requests are rejected until the service’s internal counters fall back under the configured rate window. The platform uses the standard throttling pattern: after hitting the limit, requests can be attempted again only after the Retry-After time has passed. There is no separate documented daily limit in the provided information, but repeated retries while still within the same rolling window will continue to be counted and rejected.

Key points based on the documented behavior:

1. Email throttling is enforced per minute and per hour.
2. When the limit is exceeded, the service returns 429 and the request must be retried only after the indicated Retry-After period.
3. All requests (including repeated retries) accrue against usage limits, so immediate or frequent retries can prolong the throttling period.
4. If sustained volume is needed beyond the default limits, the guidance is to request a quota increase for email sending limits via Azure Support for eligible scenarios. However, higher limits are not available for Azure-managed domains; they are only available for verified custom domains.

Given this, if 429 responses continue for several hours:

• It is likely that ongoing retries or additional send attempts are keeping the usage within the rolling window’s threshold.
• Ensure the application reduces send frequency and respects Retry-After headers.
• For higher sustained throughput, consider moving to a verified custom domain and then requesting a quota increase as documented.

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References:

• Service limits for Azure Communication Services
• Throw an exception when email sending tier limit is reached (programming-language-python)
• Throw an exception when email sending tier limit is reached (programming-language-csharp)
• Throw an exception when email sending tier limit is reached (programming-language-java)
• Throw an exception when email sending tier limit is reached (programming-language-javascript)

Answer 2

Hi @Lauren May

Thank you for reaching us regarding the issue
The behavior you are experiencing is expected due to how Azure Communication Services (ACS) Email handles rate limiting (throttling).

ACS Email enforces per‑minute and per‑hour sending limits, and when these limits are exceeded, the service returns HTTP 429 (Too Many Requests).

Once throttling is triggered:

• There is no fixed “cool‑down period”. Instead, ACS uses rolling time windows for enforcing limits.
• Any additional requests (including retries) continue to count toward the quota, which can keep the service in a throttled state.
• Microsoft recommends avoiding immediate or frequent retries, as they prolong throttling by consuming the remaining quota.
• By design, the SDK may automatically retry after a delay, which can also contribute to continued 429 responses if not controlled.

Recommended actions:

• Respect the Retry-After header returned in the response before sending new requests.
• Implement retry/backoff logic (for example, exponential backoff).
• Temporarily stop sending requests completely for some time to allow the rolling counters to fall below the limit.
• Ensure there are no other ACS Email resources in the same subscription contributing to the usage.

Reference:
https://learn.microsoft.com/en-us/azure/communication-services/concepts/service-limits#email

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