# `mx_unreachable`: the mail exchangers could not be reached from the verification network

The probe could not reach any of the domain's mail servers: unreachable at the network level rather than a refused connection.

- 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.

Transient routing problems and geo-restricted mail servers produce it. The answer is not cached for long, so a later lookup probes again.

**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": "mx_unreachable",
    "mailbox_exists": null
}
```

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