Email verification looks simple on the surface, but in practice it fails more often than many teams expect.
From what I have seen, most issues are not caused by the email service itself, but by how verification systems are designed and tested.
Common failure points:
Domains that are silently blocked
Messages delayed just long enough to expire
OTP codes with very short lifetimes
Rate limits that are easy to hit during retries
Users refreshing or resubmitting forms too quickly