Sometimes, an email verification can’t be completed, and an address returns an “Unknown” status or a verification failed message in Hunter.
This usually happens when the recipient’s mail server is unavailable, blocks verification attempts, or uses strict privacy settings.
Understanding why an email can’t be verified helps you interpret your results correctly, protect deliverability, and decide whether to retry verification or remove the address from your list.
When and why email verification might fail
Most email addresses in Hunter are already verified, and for users with All in one outreach paid plans, Hunter automatically re-verifies saved leads every month at no extra cost.
However, some addresses can’t be fully verified due to external restrictions or temporary server issues.
💡 You can verify emails in several ways:
individually with the Email Verifier,
in bulk with the Bulk Email Verifier,
directly from the Leads section,
or automatically through the Email Verifier API.
If you’re interested in understanding the technical process behind these checks, see What checks are performed during the email verification process?.
If a verification fails, you’ll usually see one of the following causes.
Common causes of why an email can't be verified
Below are the most frequent causes for a failed verification or “Unknown” result, and what you can do next.
1. Accept-All (Catch-All) domains
1. Accept-All (Catch-All) domains
Some mail servers are configured to accept all incoming messages, even if the mailbox doesn’t exist.
Because the server never confirms whether an address is real, verification tools (including Hunter) can’t determine validity with full accuracy.
💡 What to do: Use the Confidence Score to estimate deliverability. A score above 85% suggests the address is likely valid.
→ Learn more: What does an Accept-All email status mean?
2. Temporary email server issues
2. Temporary email server issues
Sometimes, the recipient’s mail server is unavailable, overloaded, or slow to respond.
When that happens, Hunter can’t complete the verification, resulting in an “Unknown” status.
💡 What to do: Try verifying again later. Once the server becomes reachable, the verification may succeed.
No credits are deducted until the check is actually completed.
3. Email server blocking verification requests
3. Email server blocking verification requests
Some organizations, especially corporate, government, or educational domains, block external verification attempts for security reasons.
In these cases, Hunter can’t confirm the mailbox’s existence, and the status remains “Unknown.”
💡 What to do: Since the server blocks all attempts, retrying won’t help. It’s best to exclude these addresses from your outreach list.
No credits are deducted when an email cannot be verified due to an "unknown" status.
4. Security filters and anti-spam protection
4. Security filters and anti-spam protection
Strong anti-spam systems may detect verification attempts and block them automatically.
This can cause verification to fail even for valid addresses, returning an “Unknown” status.
💡 What to do: You can try re-verifying later, sometimes filters reset after a few hours. If the result stays “Unknown,” avoid using that email for campaigns.
No credits are deducted when an email cannot be verified due to an "unknown" status.
How credits are used when verification fails
No credits are deducted when a verification can’t be completed (Unknown).
Credits are used when a verification result is successfully returned (Valid, Invalid, or Accept-All).
You can check your remaining credits in your Hunter dashboard anytime.