What does "Unknown" status mean and why do I see it?

Unknown means we ran the verification but the recipient mail server did not give us a definitive answer. The address might be real, might be invalid, might be a catch-all — we cannot tell from the response we got. In our scoring it shows as 0.

It is the verifier equivalent of "the phone rang but nobody answered." The line is there; whether there is a working phone at the end of it is a different question.

Why this happens

• The mail server timed out. Common with overloaded or under-provisioned servers.
• The server temporarily rejected our connection (often a soft-bounce code like 421 or 451).
• Greylisting. The server deliberately rejects unfamiliar senders the first time and accepts the retry — but if our retry also times out, the result stays inconclusive.
• An anti-spam system blocked our verification probe before it could complete.
• A mail provider that does not give honest answers to SMTP checks (some large providers deliberately respond to every probe with the same generic reply, regardless of whether the mailbox exists).

What we do about it

Two safety nets:

1. If our primary verification engine returns unknown, we automatically retry through a second engine before finalizing the result. Most unknowns from one engine become a definitive answer with the second.
2. If both engines return unknown, the credit is automatically refunded to your account. You only pay for definitive results.

In practice, this means a clean run on a normal list usually produces a small single-digit percentage of unknowns. Lists with a high concentration of corporate domains (which often run aggressive anti-probe defenses) produce more.

What to do with unknown results

If unknowns are a small portion of your list, you have two options: skip them on this send and try again next month (mail server state changes), or send to them and watch the engagement closely. If they bounce, remove them. If they open, mark them as valid and keep them.

For high-stakes sends (cold outreach to important prospects, important transactional flows), skip unknowns entirely. For lower-stakes sends (newsletters to engaged subscribers), the risk of including them is small.