Mailbox exists, is reachable, and not known to be illegitimate or disposable.
The user mailbox has been disabled.
The user mailbox does not exist at this domain.
The user mailbox is full.
The syntax of the email address is incorrect.
The domain is not responding to validation requests or does not have any active mail servers.
The domain cannot be resolved.
Seed, spamtrap, black hole, technical role account or inactive domain.
Role accounts such as support@, sales@, info@.
The domain was close to a common domain and although it exists, it is highly unlikely to be correct.
Belongs to a disposable email address provider.
The address doesn't appear to be nefarious (as far as we know), but we can't determine if it is deliverable or not.
The request timed out due to the host domain not responding in time.
The domain is accept-all, so the username cannot be validated.