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Mailbox handshake

published · September 10, 2026

A mailbox handshake is an SMTP conversation opened with a domain's mail server as far as the RCPT TO step and abandoned before any message is sent.

A mailbox handshake is an SMTP conversation opened with a domain's mail server as far as the RCPT TO step and abandoned before any message is sent. The server's response to that one command is what says whether a mailbox exists. No mail is transmitted, so nobody receives anything.

What Spaw answers

smtp_checked says whether the handshake ran, smtp_checked_at when, mailbox_exists what it established, and smtp_reason why it could not establish anything when it could not — greylisted, timeout, no_connect, mx_unreachable, provider_unavailable and the rest.

When it runs, and when it does not

The handshake is the only part of an email lookup that leaves this server, so it is gated deliberately. It runs only when the domain's MX accepts mail and no free signal has already settled the address: a disposable domain, a typo-squat or a local part the provider's own rules reject is answered from DNS and open lists alone and never leaves Spaw.

A mailbox the handshake could not confirm is reported risky with the reason unverified. It is never reported deliverable — that word is reserved for a mailbox a server confirmed.

Related

markdown version: /glossary/mailbox-handshake.md

Verify addresses the same way

The Spaw API runs every check described here on each lookup, with an SMTP handshake that never sends mail. 10 free lookups a month, no card required.

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More glossary

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  • Fictional number

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  • International (exit) prefix

    An international prefix is what is dialled to leave a country before the calling code: 00 in most of the world, 011 in North America. Spaw reads it and suggests a fix in did_you_mean.

  • International network number

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  • Live carrier check (HLR lookup)

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  • Number portability

    Number portability lets a subscriber keep a phone number when changing carrier. The number's block stays listed under the carrier it was issued to, so block data names the original holder.