Definition
SMTP (Simple Mail Transfer Protocol, RFC 5321) is how mail servers hand messages to each other. Before any content is sent, the two servers have a short conversation:
S: 220 mx.example.com ESMTP ready
C: EHLO verifier.example.net
S: 250-mx.example.com Hello
C: MAIL FROM:<[email protected]>
S: 250 2.1.0 OK
C: RCPT TO:<[email protected]>
S: 250 2.1.5 OK
C: QUIT
The important line is RCPT TO. The client names a recipient and the server says whether it will accept mail for that address. A 250 reply means yes. A 550 5.1.1 reply usually means the mailbox does not exist.
The old VRFY command was meant to answer "does this user exist?" directly, but most servers disable it because spammers abused it.
Why it matters
The RCPT TO step is the most direct way to learn whether a mailbox exists without emailing it. Mailbox-level verification is built on it: connect to the domain's mail server, go as far as RCPT TO, read the answer, and quit before sending any data.
It has limits:
- Catch-all servers answer
250for every address. - Greylisting servers answer "try later" to strangers.
- Some providers accept at RCPT TO and reject later.
- Security gateways may block or tarpit verification traffic.
Running these checks yourself from a single server at volume is a quick way to get that IP blocked, and many cloud providers block outbound port 25 by default.
How SimpleCold handles it
The mailbox check is one step in SimpleCold's pipeline, and it only runs after syntax, domain, MX, disposable, role and catch-all checks. Connections go through dedicated verification infrastructure, not your sending servers. The server's answer is mapped to a status such as mailbox_found, mailbox_not_found or blocked_by_server, with a plain reason, and the failType field shows whether the failure is permanent or temporary.
Check one address with the free email checker.
Related reading
- How SMTP email verification works
- How to verify an email address without sending an email
- Email verification API