Verdict reasons
Every answer leads with a verdict: deliverable, risky, or undeliverable. When the verdict is not deliverable, the reason field says why in one machine-readable word, and smtp_reason adds the detail behind the mailbox handshake. Each value has its own page with the full explanation, an example, and what to do with the address.
Undeliverable reasons
Do not send. The risk score is always 100, the address joins your suppression list, and the lookup is free.
| reason | meaning |
|---|---|
| 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_not_found | the mail server rejected the mailbox during the handshake |
| mailbox_disabled | the mailbox exists but its provider has disabled it |
| suppressed | the address is on your account's suppression list |
Risky reasons
Mail may arrive, but something argues for caution. Each reason adds its published weight to the risk score; the answer costs 1 credit like any deliverable one.
| reason | meaning | risk weight |
|---|---|---|
| likely_typo | the domain is a known misspelling of a real mail provider | +60 |
| disposable | the domain belongs to a disposable, temporary email provider | +80 |
| implicit_mx | the domain has no MX record and relies on the address-record fallback | +30 |
| mailbox_full | the mailbox is over its storage quota | +20 |
| role | a shared role inbox such as info@ or support@ rather than a person | +30 |
| catch_all | the server accepts every address at the domain, so the mailbox cannot be confirmed | +30 |
Mailbox-check details
The smtp_reason field explains what the SMTP handshake found, or why it could not settle the mailbox. A message is never sent.
| smtp_reason | meaning |
|---|---|
| mailbox_full | the mailbox is over its storage quota |
| no_mailbox | the server answered that the mailbox does not exist |
| 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 |
Every page here has a Markdown twin at the same URL with a .md suffix, and all of them are collected in llms-full.txt.