How SMTP verification works

After DNS finds the domain’s mail exchangers, NexiphorVerifier connects to an MX host, speaks SMTP (HELO verify.nexiphorverifier.com), and issues RCPT TO for the address. MAIL FROM is probe@nexiphorverifier.com.

A 250-class accept after a same-session mailbox-specific random reject is Valid: the provider accepted this recipient under a discriminating probe. That is not proof the mailbox exists, and it is not a delivery guarantee. A 550-class reject is Invalid. An accept on a domain that takes any local part is cannot distinguish — status Risky, not Valid, and not “dangerous.” An accept that does not confirm the mailbox is a different claim. A mailbox that cannot receive now is indicated, not absent. Timeouts, greylisting that never clears, and blocks are Unknown.

What SMTP is not

It is not inbox placement, warmup, or a guarantee the user will read the mail. It is evidence about the mailbox at probe time.

Product framing: Why NexiphorVerifier.