CleanContact resolves every address to exactly one of three statuses. Each comes with a detail string explaining the reasoning, but the status is what you act on.
Good
The receiving server confirmed the mailbox exists and accepts mail. These are safe to send to. In practice this is the bulk of a healthy list.
Bad
The server explicitly rejected the address — it does not exist, has been disabled, or is otherwise refused. Remove these. Sending to them is exactly the behaviour that damages reputation.
Unknown
The server would not give a definitive answer in the allotted time. This is not a failure of the address — it is a property of the server. Common causes:
- Greylisting — the server deliberately delays unfamiliar senders and asks them to retry.
- Catch-all configuration that accepts everything at the door regardless of the mailbox.
- Temporary rate limiting or a slow response beyond the timeout.
Treat Unknown with caution: retry later, send to it in a low-risk warm-up segment, or exclude it from aggressive campaigns. Do not treat it as Bad — many Unknowns are perfectly deliverable.