02Verify API
Email verification API — endpoints, verdicts, and what a response looks like
Two endpoints: one address at a time, or up to a hundred in a single call. Both run the same syntax → MX → SMTP pipeline and return one of four verdicts with the reason attached — no confidence score to reverse-engineer.
API in early access — endpoints and fields below are current as documented in RankFabrik's catalogue, checked 14 September 2026.
Single address — GET /verifier
One required parameter — the email — and one option, sonder. With
sonder=false, verification stops at the MX record: faster, and the verdict is more
cautious. With sonder=true (the default), it runs the full SMTP probe.
GET /[email protected]&sonder=true
{
"email": "[email protected]",
"verdict": "valid",
"code": "mailbox_exists",
"detail": "SMTP accepted RCPT TO",
"mx": "aspmx.l.google.com",
"generique": false,
"source": "smtp"
}
A catch-all domain — one that accepts every address at RCPT TO, confirmed or not — comes back
risky, flagged, never silently upgraded to valid:
{
"email": "[email protected]",
"verdict": "risky",
"code": "catch_all_domain",
"detail": "Domain accepts all recipients at RCPT TO",
"mx": "mx.bigcorp.com",
"generique": true,
"source": "smtp"
}Batch — GET /verifier-lot
Up to a hundred addresses in one call, deduplicated on receipt, with a per-verdict summary — see the bulk verifier page for the full billing and truncation behavior.
GET /verifier-lot?emails=a@…,b@…,c@…
{
"recus": 3,
"count": 3,
"summary": { "valid": 2, "risky": 1 },
"results": [
{ "email": "a@…", "verdict": "valid", "code": "mailbox_exists" }
],
"truncated": false
}Why a verdict beats a score
| Reason | What it looks like as a verdict |
|---|---|
| Mailbox confirmed at RCPT TO | valid |
| No such mailbox, bad syntax, or no MX records | invalid |
| Catch-all domain, or a known disposable provider | risky, and generique: true for catch-all |
| Mail host refuses the verification probe | unknown, with a third-party fallback consulted when configured |
Get access
Questions
How can I verify an email address without sending an email?
An SMTP probe opens a connection to the domain's mail server and issues RCPT TO for the address, then closes the session before DATA — the command that would actually transfer a message. The server's response to RCPT TO (accepted, rejected, deferred) is enough to settle a verdict without the recipient ever receiving anything.
Is there a free API that can verify addresses?
RankFabrik's Verify API is in early access at https://rankfabrik.com/produits/verification. For one-off lookups without an API key, the free tool at https://rankfabrik.com/email-checker runs the same syntax/MX/SMTP pipeline.
Is there a free email API?
The checker at https://rankfabrik.com/email-checker covers single-address lookups with no key required. The full Verify API — batch verification, a fast <code>sonder=false</code> mode, per-verdict summaries — is in early access at https://rankfabrik.com/produits/verification.
Is there a free email OTP API available?
That's a different product: a one-time-password API sends and validates a login code, while Verify only checks whether an address exists and can receive mail. RankFabrik doesn't send OTP codes — it settles whether an address is worth sending to in the first place.