Intermittent 404 DomainNotLinked on a linked, verified custom domain since 2026-10-01 (most sends fail)

Hi all! Could someone give me some light into a failure situation with ACS email sending?

Since 2026-10-01 06:30 UTC, most email sends from our Azure Communication Services resource

  (data location UK) fail with HTTP 404, x-ms-error-code DomainNotLinked:

  "The specified sender domain has not been linked."

  Some identical sends still succeed with 202. Before 1 October we had no failures at all.

  What we have checked (via the portal and the ARM API):

  - The custom domain is listed in the Communication Services resource's linked domains

    (provisioningState Succeeded).

  - Domain, SPF, DKIM and DKIM2 are all Verified on the Email Communication Service

    (CustomerManaged domain).

  - The sender usernames exist (DoNotReply and a custom one).

  - No configuration changes to either resource in the activity log since 30 September.

  - Service Health shows no incident for our subscription.

  - Our code has not changed (Python azure-communication-email 1.0.0, EmailClient.begin_send).

  Failure rate:

  - Production, 1 October (UTC): 06-09h 10 ok / 11 failed; 09-12h 25 / 59; 12-15h 36 / 56; 15-18h 16 / 27.

  - A manual test at about 18:50 UTC, 10 sends alternating the DoNotReply@ and the custom sender

    on the same domain: 9 failed, 1 succeeded. So it is not tied to the sender username.

  Impact: login verification codes and report emails to our users are not delivered on most attempts.

  Is there a known platform issue with Email Communication Services domain linking in UK, or                                                                                             

  could the service-side link state for our resource be inconsistent? We can share resource

  and request IDs privately.