Glossary

What does an “unknown” verification result mean?

An unknown verification result means the mailbox was never tested — the probe was blocked, DNS failed, the server deferred, or the domain accepts every address. It is not a soft undeliverable; nothing was established about this mailbox either way.

Also known as Unknown verdict · Inconclusive verification · Unverifiable address

Why there are three verdicts and not two

A caller has exactly one decision to make about an address: send, do not send, or ask again. Three verdicts, one each. deliverable — the mail server confirmed the mailbox. undeliverable — the mail server said it does not exist. unknown — the question was not answered.

A false undeliverable is the most expensive mistake in this category, and it is invisible. An address marked invalid is removed from a list permanently. If the mailbox was real and the verifier simply could not reach it, nobody ever finds out: there is no bounce, no complaint, no signal at all. The customer just stops hearing from you.

That is why a two-verdict model is not a simplification. It is a decision to guess on the customer’s behalf and not tell them.

The five things an unknown actually means

Every unknown carries a reason, and the reasons are not interchangeable. Two of them are answers about the domain that will not change; three of them are retryable: true, meaning the mailbox is still worth asking about later.

How VerifyInbox handles it

Every result carries reason, message and retryable, plus the three checks the verdict was built from — format, domainStatus and mailboxStatus. There is no opaque score: the earlier score, confidenceBand and bounceRateEstimate fields were removed because they were an opinion dressed as a measurement. An integration that switches on retryable gets every one of these cases right without knowing anything about SMTP.

The reasons an unknown carries, and what to do with each
`reason`	What happened	`retryable`	Charged
`ip_blocked`	The receiving server refused us, not the address	`true`	No
`dns_error`	The domain’s DNS did not answer — a SERVFAIL or a timeout	`true`	No
`greylisted`	The server asked us to come back later	`true`	No
`catch_all`	The domain accepts every address, so its 250 says nothing about this mailbox	`false`	Yes
`mailbox_full`	The mailbox exists but is over quota, so mail queued to it bounces	`false`	Yes

What to do with an unknown

Never write it into your database as undeliverable. That is the one instruction on this page that matters.

If retryable is true, ask again later — nothing was established, and the condition that stopped the probe is transient by definition. If it is false, retrying will produce the same answer; treat the address as unconfirmed, segment it away from your confirmed deliverables, and send to it carefully rather than in bulk.

The same rule applies to a 503 with the error code ENGINE_CAPACITY. It means no sending capacity was available, so the work was not done at all. It is not a result and it is not charged.

What it costs

A retryable result is never charged. A blocked probe, a DNS failure or a greylist deferral bills nothing, because the mailbox was not tested and there is nothing to bill for. The credit is reserved and then released, so there is no ledger row at all rather than a charge followed by a refund.

A catch-all or full-mailbox result is charged the normal half credit. The probe did establish something — how the domain behaves, or that the mailbox is over quota — and that is what the charge is for. Saying "we never charge for unknown" would be simpler and it would not be true.

Check one address and see the SMTP evidence behind the answer. No account, and it runs now: Email verifier

Related terms

Related SMTP codes

Sources

Primary sources checked 11 September 2026. If something here is out of date, tell us and we will correct it.

Unknown verification result: questions

Why do some addresses come back as unknown?

Because the mailbox was genuinely not tested. The receiving server blocked the probe, greylisted it, or DNS failed — or the domain is catch-all, so its acceptance describes the domain rather than the mailbox. Check retryable: true means ask again, false means the answer will not change.

What does the status value mean?

One of three things. Deliverable: the mail server confirmed this mailbox. Undeliverable: the mail server said it does not exist. Unknown: nothing was established. The reason field says which of the five underlying conditions produced an unknown, and retryable says whether asking again can change it.

What should I do with risky addresses?

There is no risky verdict here, deliberately. What other tools call risky splits into two different facts: a catch-all or full mailbox is unknown, because the mailbox was not confirmed; a role or disposable address is deliverable with a flag, because the mailbox answered. Segment on the flag, not on a bucket.

Can an email sent to a "valid" address bounce?

Yes. A verification is a statement about one moment: the mailbox existed when it was probed. It can be closed, filled or blocked afterwards, and a catch-all domain can accept at the door and reject later. That is why the result carries the evidence rather than a promise.

Why do verified emails still bounce?

Usually one of three reasons: the address was verified long enough ago for the mailbox to have closed; the domain is catch-all, so acceptance was never a confirmation; or the rejection is about the sender rather than the recipient, such as a policy or reputation block. The first is fixed by verifying closer to the send.

How accurate is email verification?

It depends entirely on whether the mail server answered, which is why the result says so. A confirmed deliverable is close to certain at the moment it was checked. An unknown is not a degraded deliverable — it is the absence of an answer, and any tool that reports it as a number is inventing one.