Why some results are unknown

Unknown means we asked and didn’t get a definite answer. We never guess: an address we can’t confirm either way stays Unknown rather than being called valid or invalid. And because it isn’t an answer, you don’t pay for it — unknown results are refunded automatically when the job finishes.

Common reasons

• The server answered “try again later” and kept saying it. Some servers do this on purpose to slow down unknown senders (greylisting). We wait and retry, but only a few times.
• The server didn’t respond in time, even after retries with longer timeouts.
• The server refused to answer us — for example a policy that only accepts mail from known senders. That’s about the server, not the mailbox.
• The server said yes, but the domain’s catch-all test couldn’t be confirmed. A “yes” only counts if we know the domain doesn’t say yes to everything, so these stay Unknown (Server accepted but could not be confirmed).
• Microsoft 365 had no record. Microsoft’s directory doesn’t list aliases, shared mailboxes or groups, so “no record” there isn’t proof the mailbox is missing. We follow up with a mail-server check; if that can’t settle it, it stays Unknown.
• The domain’s DNS didn’t answer. A failed lookup is never treated as “no mail server”.
• The domain is parked with a domain seller.
• We deliberately didn’t check it — administrative addresses like abuse@ and postmaster@ are never probed.

Not Verified is different: it means we never got to check the address at all. That’s on us, and it’s refunded too.

What to do with unknowns

• Run them again later. Export the Unknown category from the Breakdown tab and upload it as a new list in a day or two. Many servers that were busy or cautious will answer next time.
• Treat them as risky if you need to send now: in a separate, lower-volume campaign, or not at all.

If a large share of a list comes back Unknown, tell us in the chat — we look into it.