How SMTP Mailbox Verification Works

For most domains, the only authority on whether a mailbox exists is the mail server that hosts it. CleanContact asks that server directly, using the same SMTP protocol a real mail client uses — but it never actually delivers a message.

Finding the server

First we look up the domain's MX records to learn which servers are responsible for its mail, and connect to the highest-priority one.

The handshake

We then walk through the opening of an SMTP transaction:

220 mx.example.com ESMTP
HELO checker.cleancontact.ru
250 mx.example.com
MAIL FROM:<probe@cleancontact.ru>
250 OK
RCPT TO:<someone@example.com>
250 OK   ← mailbox exists
... or ...
550 No such user   ← mailbox does not exist

The decisive moment is the response to RCPT TO. A 250 means the server is willing to accept mail for that address; a 550 (or similar 5xx) means it will not. We read that response and then disconnect — no DATA, no message, nothing lands in the inbox.

Why it is not always clean

Servers do not have to be honest. A catch-all accepts every RCPT TO regardless of whether the mailbox is real, and greylisting returns a temporary 4xx asking us to come back later. CleanContact handles these by routing to fresh nodes, respecting cooldowns, and returning Unknown rather than guessing.