SMTP verification
SMTP verification checks whether a mailbox exists by starting a delivery conversation with the mail server and stopping before any message is sent.
How it works
The checker connects to the domain's mail server, names the recipient and reads the answer. A refusal suggests the mailbox does not exist; an acceptance suggests it does.
Results can mislead. Catch-all servers accept everything, some servers delay or hide their answers, and providers that throttle unknown senders can block the check. A good verifier reports these cases as unknown.
Run it near the end of a verification, after the cheaper syntax and domain checks, because it is the slowest step and the one most likely to be rate limited. Space the connections out, never send a message body, and read a temporary failure as unknown and not as invalid so that a good address is not thrown away.
Related LookValid products
| Product | Credits | Docs |
|---|---|---|
| Email Validation (mailbox verification) | 1 credit per check | Product page |
New to the API? Start with the quickstart or the API reference. Agents can call the same lookups through the MCP server.