When a product supports email signup plus Facebook-based identity flows, the backend usually looks stable long before the test evidence does. I have seen signup checks pass because an old verification email was still sitting in a shared inbox, while the current API run had actually failed to enqueue anything. The code path looked green. The user path was not.
For teams testing temp mail for facebook scenarios, I think the safest pattern is to treat the inbox as part of the API contract. A signup request should produce one clearly attributable verification message, tied to one request window, with enough metadata to prove the email belongs to the current run. That sounds strict, but it keeps auth debugging from turning into inbox archaeology.
Why Facebook signup email tests go flaky
The flakiness is usually not in Facebook itself. It is in the boundary between your POST /signup flow and the mailbox used to verify the result.
Common failure modes I keep seeing:
