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 mailbox-specific evidence is Valid. A 550-class reject is Invalid. An accept on a domain that takes any local part is a catch-all → Risky. 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.