What email verification is
Email verification checks whether an address can receive mail before you send to it. It is not a spam filter and it is not enrichment: the single question it answers is whether a message to that address will land in a mailbox or bounce.
This matters because mailbox providers judge senders partly on bounce rate. A list with a few percent of dead addresses can pull an entire domain's reputation down, which pushes even valid recipients into spam.
How the checks work
- Syntax. The address is parsed against RFC 5322 rules — one @, valid local part, valid domain label structure.
- Domain and MX. DNS is queried for mail exchange records. A domain with no MX (and no fallback A record) cannot receive mail at all.
- SMTP mailbox probe. The verifier connects to the mail server and asks whether the recipient exists, then disconnects before sending data.
- Risk classification. The address is flagged as disposable, role-based (info@, sales@), free-provider or catch-all so you can decide policy per segment.
Provider behaviour differs widely — see the domain lookup directory for how individual providers respond.
Reading the results
- Valid — the mailbox exists and accepted the probe. Safe to send.
- Invalid — the server rejected the recipient. Remove it; this is the hard-bounce category.
- Accept-all / catch-all — the domain accepts everything, so existence cannot be proven. Send with care and watch engagement.
- Disposable — a temporary inbox. Usually worth blocking at signup rather than mailing.
- Role — a shared function address. Deliverable but often a complaint source in marketing sends.
- Unknown — the server would not answer, typically because of greylisting or rate limiting. Retry later before deciding.
Cleaning a bulk list
Export your list as CSV, keep every original column, and upload it. A good bulk run returns the same rows in the same order with verification columns appended, so you can re-import into your ESP without remapping fields.
Then split the file: mail the valid segment, drop the invalid segment, and treat accept-all as a separate low-volume test group before adding it to your main sends.
Verifying through an API
The highest-value place to verify is the signup form, not the campaign. A single API call at registration keeps dead and disposable addresses out of the database entirely, so nothing downstream has to clean them later.
Full endpoints, response fields and error codes are in the API documentation, with runnable examples in the developer hub.
How often to re-verify
Contact data decays as people change jobs and abandon mailboxes. Re-verify active lists every three to six months, always verify before a re-engagement campaign, and verify continuously at the point of capture.
Frequently asked questions
- What is email verification?
- Email verification is the process of checking whether an email address exists and can receive mail, without sending a message to it. It combines syntax checks, domain and MX record lookups, and an SMTP conversation with the receiving mail server.
- Does verifying an email send an email?
- No. A verifier opens an SMTP session and asks the receiving server whether the mailbox exists, then disconnects before any message data is transmitted. The mailbox owner sees nothing.
- What is a catch-all domain?
- A catch-all domain accepts mail for every address at that domain, whether or not the mailbox exists. SMTP cannot prove a specific address is real on such a domain, so results are returned as accept-all rather than valid.
- How often should a list be re-verified?
- Email data decays roughly 2 to 3 percent per month. Re-verify active sending lists every 3 to 6 months, and always verify a list that has been dormant for more than six months before a campaign.
- Will verification fix my deliverability?
- Verification removes the hard-bounce driver of poor reputation, which is usually the biggest single factor. Authentication (SPF, DKIM, DMARC), sending cadence and content still matter.