# `mailbox_not_found`: the mail server rejected the mailbox during the handshake

The domain accepts mail, but at the RCPT TO step the server answered that this mailbox does not exist. No message was sent.

- Appears in: `reason`
- Verdict: undeliverable
- Billing: Free. Undeliverable verdicts never cost a credit.

The SMTP conversation reached the recipient check and the server rejected it, typically with a 550 5.1.1 "user unknown" reply. The handshake ends there; no DATA is ever transmitted and nothing lands in an inbox or a spam folder.

The verdict is undeliverable, mailbox_exists is false, and smtp_reason carries the detail, usually no_mailbox. The address is also added to your account's suppression list, so batch and bulk runs answer it for free until it is explicitly re-verified.

**What to do.** Remove the address from the list. A confirmed missing mailbox is the single strongest bounce predictor, and sending to it damages sender reputation.

```json
{
    "email": "nobody@acme.com",
    "deliverable": "undeliverable",
    "reason": "mailbox_not_found",
    "mailbox_exists": false,
    "smtp_checked": true,
    "smtp_reason": "no_mailbox"
}
```

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