Short answer: unknown means we could not obtain a reliable signal to confirm the address one way or the other. It does not mean the address is bad. Unknown results are never charged.
Why an address comes back as unknown
unknown is returned whenever the receiving mail system prevents us from getting a trustworthy answer. The most common causes:
- Anti-verification providers — Apple (iCloud) and personal Microsoft (Outlook/Hotmail/Live) deliberately hide mailbox existence, so we skip probing and return
unknownby design. - Greylisting — the server temporarily defers the check and asks us to try later.
- Rate limiting — the provider throttled verification traffic at that moment.
- Timeouts / blocks — the server was slow to respond, or blocked the verification connection.
In every one of these cases, the honest answer is "we can't tell" — so we return unknown rather than guessing valid or invalid.
How to treat unknown addresses
Think of unknown as "unconfirmed", not "failed." A practical rule of thumb:
- If the address came from a legitimate signup, purchase, or opt-in, it is usually deliverable — sending is generally safe.
- If the address came from a scraped or purchased list with no prior relationship, treat it with more caution.
- The only definitive test for these providers is a real send with normal bounce monitoring.
Do not automatically delete unknown addresses as if they were invalid — you would be discarding many genuinely deliverable contacts.
Billing
Unknown results are never charged. You only spend credits on results we can actually determine (valid, invalid, catch-all, role).