---
title: "What is SMTP verification?"
url: "https://lookvalid.com/glossary/smtp-verification.md"
canonical_url: "https://lookvalid.com/glossary/smtp-verification"
last_updated: "2026-10-23"
type: "conceptual"
summary: "SMTP verification checks whether a mailbox exists by starting a delivery conversation with the mail server and stopping before any message is sent."
related: ["https://lookvalid.com/glossary/catch-all.md", "https://lookvalid.com/glossary/mx-record.md", "https://lookvalid.com/glossary/hard-bounce.md"]
---

# What is 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.

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

- [What is Catch-all?](https://lookvalid.com/glossary/catch-all.md)
- [What is MX record (Mail Exchanger record)?](https://lookvalid.com/glossary/mx-record.md)
- [What is Hard bounce?](https://lookvalid.com/glossary/hard-bounce.md)