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greylisted

The server deferred the handshake and asked the sender to come back later

Greylisting answered the probe with a temporary failure. The mailbox is left unverified and the verdict rests on the free signals.

appears in
smtp_reason
credits
Free: an answer the mailbox check could not settle costs nothing and leaves no 7-day repeat marker; the settling re-checks bill 1 credit only when they confirm the mailbox. A verdict another signal decides, such as role, bills 1 credit as that verdict.

What it means

Greylisting is an anti-spam technique: the first connection from an unknown sender gets a temporary 4xx failure and is expected to retry minutes later. A verification probe cannot wait that long, so smtp_checked is false and mailbox_exists is null.

The verdict is risky with reason unverified: the unverified weight of 10 is added to the risk score and a mailbox_confidence estimate is attached, built from everything else Spaw knows about the address.

What to do

Pass callback_url with the lookup: Spaw re-checks the address after 5 and 20 minutes and pushes the settled verdict to you, billing 1 credit only if a re-check confirms the mailbox. Or re-verify later: an unverified answer leaves no repeat marker, so once its 15-minute probe result has expired the next lookup asks the server again.

Example response 

{
    "deliverable": "risky",
    "reason": "unverified",
    "smtp_checked": false,
    "smtp_reason": "greylisted",
    "mailbox_exists": null,
    "mailbox_confidence": 65
}

Abbreviated: a real answer carries all 46 fields, listed in the verify-email reference.

Related

markdown version: /docs/reasons/greylisted.md