What's the best way to use the Unknown auto-refund in production?

The Valid Email Checker engine returns unknown when it cannot reach a definitive answer. Every Unknown result auto-refunds the credit — you were never charged for the inconclusive check. That refund is reflected immediately in your Credits History and is non-negotiable, so the production question is not whether to handle it, but how to act on it.

Why Unknown happens

Common causes of an Unknown result:

• The receiving SMTP server is rate-limiting our probe.
• Greylisting at the recipient domain temporarily refused the handshake.
• A transient network or DNS failure during the verification.
• The domain is using an aggressive antispam system that holds the SMTP conversation open without answering definitively.

None of these mean the address is bad. They mean we could not check it cleanly right now. Forcing a definitive answer here would risk false negatives, which we refuse to do — see what does Unknown status mean for the full engine behaviour.

Production rule 1: never block a real user on Unknown

If your signup form gets an Unknown back from the API, accept the signup. The credit was refunded. Blocking the user means you turn away real subscribers because a downstream mailbox was momentarily slow. That is the opposite of why you wired up verification in the first place.

Production rule 2: retry on a separate schedule

Capture Unknown addresses in a queue or table and re-verify them 24 to 48 hours later in a batch job. The vast majority resolve to a definitive status on the second attempt. Greylisting clears, rate limits reset, transient failures pass. Two retries are usually enough; if an address comes back Unknown three times across multiple days, treat it as practically Risky.

Production rule 3: do not retry inside the same second

Hammering the same address with five back-to-back API calls because the first one returned Unknown will just produce five Unknowns. The conditions that caused the first one are still there. Space retries by hours, not milliseconds.

Verifying the refund landed

Open Credits History and filter by transaction type refund. Every auto-refund from an Unknown verification logs a row. The net charge for that verification is zero, which you can confirm by matching the original charge row to its refund row. See refunds and credit returns for the full refund flow.

Budgeting around Unknown

On a healthy list, Unknown rates sit around 1 to 3%. Plan your credit budget around the conclusive verification volume — the Unknown cohort is effectively free. The one budget consideration is the retry: each Unknown that retries to a conclusive status on the second attempt does charge for the second attempt. Roughly a 2% Unknown rate means a 2% extra credit spend if you retry every Unknown once.