# `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`
- Billing: Billed by the verdict it produced: a deliverable answer with smtp_checked false is 1 credit. The settling re-checks are free.

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 comes from everything else Spaw knows about the address, usually deliverable with the unverified weight of 10 added to the risk score and a mailbox_confidence estimate attached.

**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 at no cost. Or re-verify later; repeats inside seven days are free.

```json
{
    "deliverable": "deliverable",
    "smtp_checked": false,
    "smtp_reason": "greylisted",
    "mailbox_exists": null,
    "mailbox_confidence": 65
}
```

Reference: https://spaw.co/docs/reasons/greylisted