Why unknown results are free
An unresolved check should remain unresolved, and MailOK does not charge for that result.
MailOK doesn't charge for unknown results. The check hasn't given you a conclusion about the address, so it isn't a billable result.
A server may time out, temporarily refuse the request or decline to reveal useful information. That leaves the address unresolved. It doesn't make the person behind it a bad contact.
You still need to decide what to do next. For a new signup, a confirmation step may be appropriate. For an old contact with no recent history, hold the address for review. A later check may resolve a temporary problem; repeating the same check indefinitely won't guarantee an answer.
Don't mix unknowns into the valid total when reporting a cleaning run. Keep them visible so the person preparing the send knows what remains uncertain.
The pricing page sets out the billing terms. The status guide explains how unknown differs from the other results.
Check a list before your next campaign
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Keep a contact ID in your verification file
A simple export makes it easier to return each result to the right contact without exposing your whole CRM.
Before you connect an email verification API
Plan for unresolved checks, failed requests and changing addresses before putting verification into production.