Azure Communication Services Email: sends accepted, but held 15–80 minutes before delivery on a new custom domain

We send transactional email (one-time sign-in codes) through Azure Communication Services Email:

• A new custom domain, data location Europe.
• SDK @azure/communication-email 1.1.0 (beginSend, then pollUntilDone), authenticated with a managed identity.
• SPF, DKIM and DKIM2 are verified, the domain has an MX record, and receivers report spf=pass, dkim=pass and dmarc=pass.

The problem: many sends are accepted immediately, but the mail leaves Microsoft tens of minutes later. Others on the same day went out within seconds. While a message is held, polling its operation returns Running for more than 60 seconds. No row appears in ACSEmailSendMailOperational until the mail goes out.

Two examples from 13 Nov 2026 (UTC). "Created" is the timestamp in the Message-ID; "Handed to Exchange" is X-MS-Exchange-CrossTenant-OriginalArrivalTime:

The second message reached the recipient's server one second after the Exchange hand-off, so the delay is inside ACS. The domain is new, and that day we sent a few dozen test and sign-in emails while setting it up.

Questions:

1. Do new custom domains have sending limits or throttling that queue mail like this instead of returning 429? If so, what are the default limits, and how do we raise them?
2. Is there a log, metric or status that shows a send is queued or throttled, rather than only Running?
3. Is a delay of this length expected for ACS Email, or should sends normally leave within seconds?

Sign-in codes are only valid for a few minutes, so we need delivery within about a minute.