What is SMTP verification

SMTP verification is a way to check whether an email address exists by connecting directly to the recipient's mail server. No message is sent: the connection closes before any data is transmitted.

How it works

The validator looks up the domain's MX record, opens a TCP connection on port 25, and runs three SMTP commands:

1. EHLO — sender identification.
2. MAIL FROM — a technical sender address.
3. RCPT TO — the address being checked. This is where the server says whether it will accept the mailbox.

The DATA command is never sent. The connection closes right after the RCPT TO response is received.

Response codes

• 250 — address exists, server is ready to accept mail.
• 550 — mailbox not found or blocked; the address is invalid.
• 450 — temporary rejection, often greylisting; the server is asking the sender to retry later.

Limitations

SMTP verification is not bulletproof. Three problems come up regularly:

• Greylisting — the server returns 450 on the first request from an unknown IP. Without a retry, a perfectly valid address looks suspicious.
• Catch-all domains — the server accepts mail for any address in the domain, including ones that do not exist. A 250 response tells you nothing.
• Rate limiting — mail providers cap the number of SMTP connections from a single IP. In bulk verification runs, some requests will get rejected.

SMTP verification in uChecker

uChecker runs SMTP verification as one step in a multi-stage validation pipeline. Syntax, DNS, and MX records are checked first. After the SMTP handshake, additional signals are analyzed: catch-all status, presence in disposable email lists, and role-based addresses. The final result is a weighted deliverability score, not just a raw 250 or 550.