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
- 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 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; an address the handshake partner classifies as non-existent without a finer reason answers mailbox_not_found with no smtp_reason.
What to do
Remove the address from the list; it is on your suppression list already.
Example response
{
"deliverable": "undeliverable",
"reason": "mailbox_not_found",
"smtp_reason": "no_mailbox",
"mailbox_exists": false
}Abbreviated: a real answer carries all 46 fields, listed in the verify-email reference.
Related
- disabled · the server reported the mailbox as disabled or suspended
- greylisted · the server deferred the handshake and asked the sender to come back later
- timeout · the mail server did not answer within the probe's time limit
- no_connect · no mail server accepted a connection on port 25
- mx_unreachable · the mail exchangers could not be reached from the verification network
- delivered_recently · your own delivery report confirmed this mailbox
- provider_unavailable · the verification provider itself could not answer
- All verdict reasons
- Verdicts and the risk score
- API reference
markdown version: /docs/reasons/no_mailbox.md