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
- credits
- Free within your email fair use: undeliverable answers that needed a mailbox probe cost nothing up to max(500, your billed email answers) in a rolling 30 days, then 1 credit each, with meta.fair_use_billed true. A billed answer repeats free for 7 days.
What it means
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, bulk and monitor runs answer it for free until a later verdict or a reported delivery clears it; once it is untouched for 90 days those runs verify it afresh instead of trusting it.
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.
Example response
{
"email": "[email protected]",
"deliverable": "undeliverable",
"reason": "mailbox_not_found",
"mailbox_exists": false,
"smtp_checked": true,
"smtp_reason": "no_mailbox"
}Abbreviated: a real answer carries all 46 fields, listed in the verify-email reference.
Related
- invalid_syntax · the input does not parse as an email address
- invalid_local_part · the username breaks the rules its own provider enforces
- no_mx_records · the domain has no mail servers at all
- null_mx · the domain declares that it never accepts mail
- mx_unresolvable · the mail servers the domain points at do not exist
- mailbox_disabled · the mailbox exists but its provider has disabled it
- suppressed · the email address, phone number or postal address is on your account's suppression list
- All verdict reasons
- Verdicts and the risk score
- API reference
markdown version: /docs/reasons/mailbox_not_found.md