Why email verifiers return Unknown

Unknown is not “we think it might work.” It means the probe did not complete: SMTP timeout, DNS timeout, blocked connection, greylist that never resolved, or an infrastructure deferral.

Guessing Valid or Invalid in that situation would invent a mailbox state. NexiphorVerifier returns Unknown and refunds the credit on live keys.

Unknown is still useful

You can retry later, or exclude Unknown from a send. The ledger shows the refund. Sandbox keys return deterministic fake buckets and do not model real Unknowns the same way.