Entra External ID customer email OTP sign-up shows “code sent” but neither Gmail nor Hotmail receives it

We are testing a newly created Microsoft Entra External ID customer tenant with a sign-up/sign-in user flow configured for built-in Email One Time Passcode. The browser SPA uses MSAL Browser 5.29.0 with authorization code/PKCE, a ciamlogin.com authority and an exactly registered HTTPS callback. The app and callback load and the flow reaches Microsoft's Enter code page.

Two owner-controlled test inboxes at different providers (Gmail and Hotmail) received no verification email, including spam/junk checks. One resend to Hotmail also did not arrive. We have stopped requesting further codes.

Checked:

• User flow shows Email One Time Passcode and is associated with the browser app.
• Both user-information custom extension slots show None selected.
• The directory sign-in-log view exposed administrator portal events but no customer email-delivery details explaining the failed attempts.
• The sign-up troubleshooting panel displays error 16000. We recognise that this may be a preceding account-lookup result and have not established that it explains the missing email.
• Portal Service Health reported no events at inspection; this does not rule out a tenant-specific delivery issue.

Which diagnostics can establish whether the built-in OTP email was generated, suppressed, throttled or rejected before delivery? Are there customer-tenant provisioning or email-delivery settings we should inspect? Basic Azure support is available, so the portal does not offer us a private technical support case without upgrading.

No real tenant data is connected. We can share correlation details privately with Microsoft if a secure supported route is provided.