Unknown is a lack of confirmation
An unknown result does not prove that an address is invalid or safe to send. A mail server may temporarily refuse connections, apply greylisting, time out or prevent mailbox probing. Keep the raw reason with the result so a later review can distinguish a transient error from a confirmed rejection.
What to do next
- Check the spelling and any suggested correction with the contact. Do not silently replace the address.
- Separate inconclusive contacts from valid and confirmed invalid addresses. Do not delete them solely because of an unknown result.
- If the reason describes a temporary failure, retry later. Avoid repeating checks in a tight loop.
- Use prior consent, recent engagement and delivery history to decide whether a controlled send is appropriate. Honor unsubscribes and suppression rules.
Catch-all and unknown are different signals
A catch-all server accepts recipient addresses without establishing that each inbox exists. An unknown result means the verifier could not reach a conclusion. Both deserve review, but they describe different observations. A valid result accompanied by accept_all is still an unconfirmed individual mailbox.
Verification errors are different again
A failed tool request or unavailable service is not a mailbox classification. Retry after the service recovers; do not label a contact invalid because the tool could not complete the check.
Read the protocol, then check again
The SMTP specification distinguishes temporary 4xx replies from permanent 5xx replies. Read the complete reply before deciding how to handle a failure. Return to the free checker.