What an SMTP check actually tells you

SMTP is the protocol mail servers use to transfer email. A verifier can begin that conversation, present a recipient address, and stop before submitting a message.

Before connecting, a check can examine the address format and the domain's mail routing. An MX record identifies a receiving server. Missing MX records alone don't settle the question: SMTP defines an address-record fallback in some cases.

The recipient command is RCPT TO. Its response needs interpretation:

• 250 accepts the recipient at that stage. Later checks can still reject delivery.
• 550 rejects the request. It can mean a nonexistent mailbox, but also a policy restriction.
• A temporary failure leaves the question unresolved and may justify a later attempt.

A probe that stops before DATA submits no message body. The receiving server can still log the connection and the address being checked.

Those mechanics are described in the SMTP specification. Enhanced status codes can add detail about the reason for a failure.

A catch-all server complicates the result by accepting recipients regardless of whether an individual mailbox exists. Some servers also avoid exposing useful recipient information.

That is why a check should preserve uncertainty. Turning every acceptance into “a real person is here” or every refusal into “this mailbox doesn't exist” loses information the response never supplied.

Confirmation serves a different purpose. When a user completes a requested confirmation step, you have evidence of access to that inbox. An SMTP check alone cannot establish that.