# `no_mailbox`: the server answered that the mailbox does not exist

The detail behind a mailbox_not_found verdict: the handshake reached the recipient check and the server rejected the address outright.

- Appears in: `smtp_reason`
- Billing: The verdict is undeliverable, so the lookup is free.

The most reliable negative signal SMTP can give. The server was asked whether it would accept mail for the recipient and said no, usually with a 550 5.1.1 reply. Spaw records mailbox_exists false and the verdict mailbox_not_found.

Only an explicit "no such user" answer maps to no_mailbox. Ambiguous rejections and policy blocks are reported as unverified instead of being guessed.

**What to do.** Remove the address from the list; it is on your suppression list already.

```json
{
    "deliverable": "undeliverable",
    "reason": "mailbox_not_found",
    "smtp_reason": "no_mailbox",
    "mailbox_exists": false
}
```

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