An unknown result means the mail server didn't give a clear yes or no when we asked about the address. It's not a verdict on the address: it may well be real. Unknown results are always free.
A few unknowns in a list are normal. If a large share of a list comes back unknown, one of the causes below is usually behind it.
Why it happens
- The server was slow or busy. Some servers delay or refuse checks from addresses they haven't seen before, and accept them later. This often affects many addresses at the same company at once.
- A security gateway sits in front of the mailbox. Large companies often filter mail through a gateway that answers every check the same way, so the real mailbox can't be checked directly.
- The server was down or unreachable at the time of the check.
- The domain blocks checks entirely. Some organisations never confirm whether an address exists.
The diagnosis on each row (or the reason shown in the app) tells you which applies, for example a timeout or an unreachable server.
What to try
- Look at the domains. Sort or filter the unknown rows by domain. If most come from a few companies, it's those companies' servers, not your list.
- Check again later. Many unknowns are temporary. Re-check them after a few hours or the next day. Unknown results are never cached, so a re-check is a real new check, and it's still free unless it gets a clear answer.
- Re-verify only the unknowns. On the job's page, re-verify the list and pick the unknown status only. The preview shows how many addresses will be checked and the most it can cost before anything runs.
- Look at the domain. Domain intel shows the domain's mail provider, any security gateway and its deliverability index, for free. A very low index means the domain filters cold mail strictly, whatever the address.
Re-checking right away rarely helps. Give the server time: a few hours is usually enough.
Unknown, risky and catch-all are different
| Result | What it means |
|---|---|
| Unknown | The server didn't give a clear answer this time |
| Risky | We got an answer, but not one we can rely on |
| Catch-all | The domain accepts mail for any address, so the server's yes means nothing. Catch-all resolution can confirm the address |
What to do with unknown addresses
- Don't delete them automatically. Many are real, working addresses.
- Send to them separately. Put them in a smaller, slower send after your verified addresses, and watch the bounce rate.
- For important contacts, find another way to confirm: an email from the finder with a verified status, or a different channel such as LinkedIn.
When to contact us
If a domain you know is real keeps coming back unknown on every re-check over several days, email support@filtrat.io with the domain and a few example addresses. We'll take a look.