01Email Verification
Email verification: how to check if an email address is real
An email checker that only looks at the string — no @, no dot, invalid — catches typos. It cannot
tell you whether [email protected] actually has a mailbox behind it. That takes a second
layer: a live conversation with the domain's mail server, stopped before anything is delivered.
Endpoints and response fields below match RankFabrik's Verify API documentation, checked 14 September 2026.
Three checks, in order
Each step rules out a different failure mode. Skipping straight to the last one wastes time on addresses that were never going to pass; skipping it entirely means shipping a list you never really checked.
Step 1
Syntax
Is the address well-formed under RFC 5321 — local part, @, a domain with a valid label structure? This catches typos, not fake domains.
Step 2
MX records
Does the domain publish mail-exchanger records at all? No MX means no mailbox can exist there, full stop — settled without ever contacting a server.
Step 3
SMTP probe
A real SMTP session opens to the domain's mail server and issues RCPT TO for the address. The response — accepted, rejected, deferred — is the only signal that speaks to this specific mailbox.
Four verdicts, not a score
A confidence percentage hides its reasoning. RankFabrik returns one of four verdicts, each carrying the reason a human can act on:
| Verdict | What it means |
|---|---|
valid | Syntax, MX and SMTP probe all passed — the mailbox accepted RCPT TO. |
invalid | A hard failure at any stage — bad syntax, no MX, or the mailbox explicitly rejected at the SMTP layer. |
risky | Deliverable but uncertain — most often a catch-all domain, which accepts every address without confirming this one, or a known disposable provider. |
unknown | The SMTP probe couldn't settle it — some mail hosts refuse verification probes outright. A configured third-party fallback is consulted when one exists. |
Microsoft 365 domains are checked through the identity API instead of SMTP — no probe connection is opened against them.
Nothing is ever sent
The SMTP dialogue stops at RCPT TO, the command that names a recipient — before
DATA, the command that would actually transfer a message. The mail server's response to
RCPT TO (accepted, rejected, deferred) is enough to settle most addresses. The
recipient's inbox never sees anything, because nothing was ever queued to send.
Try it
Check one address for free, or read the API docs for batches and automation.
Questions
How can I verify if an email address exists?
Check three things in order: the address is syntactically valid (RFC 5321), the domain has MX records that accept mail, and — the only step that actually confirms a mailbox — an SMTP session reaches RCPT TO for that address and gets a response, without the message ever being sent. Syntax and MX checks alone only prove the domain can receive mail, not that this specific mailbox exists.
Is there a free email verification tool?
Yes. RankFabrik's email checker, at https://rankfabrik.com/email-checker, runs the same syntax, MX-record and SMTP-probe pipeline as the API for one address at a time, at no cost and with no account. The Verify API (early access, https://rankfabrik.com/produits/verification) automates the same checks for recurring or bulk lookups.
How to verify the legitimacy of an email address?
Legitimacy is more than 'does it exist'. Check whether the domain is a known disposable-email provider, whether the domain is catch-all (it accepts every address, so a single mailbox check proves less), and whether the specific mailbox accepts mail at the SMTP layer. RankFabrik folds all three into one of four verdicts — valid, invalid, risky, unknown — each with the reason attached, instead of a single opaque score.
How can I verify a Gmail account?
Gmail's mail servers respond to an SMTP RCPT TO probe like any other domain, so the same syntax → MX → SMTP pipeline settles most Gmail addresses directly. Google does not publish a free lookup API for checking whether an arbitrary Gmail address exists outside of that SMTP-level signal.