SMTP

SMTP is the protocol that moves email between servers. A sending server looks up the recipient domain's MX records, connects (normally on port 25 for server-to-server delivery) and holds a short text conversation: EHLO to introduce itself, MAIL FROM to give the envelope sender, RCPT TO for each recipient, DATA to transmit the message, then QUIT.

The server answers every command with a three-digit response code: 2xx for success, 4xx for a temporary failure, 5xx for a permanent one. The envelope addresses (MAIL FROM and RCPT TO) are separate from the From: and To: headers inside the message, which are defined by RFC 5322. That distinction matters for SPF and DMARC.

SMTP also defines a VRFY command for asking whether a mailbox exists, but most servers disable or ignore it. Mailbox checks therefore rely on how a server answers RCPT TO.

Most delivery problems a sales team sees, such as bounces, deferrals and blocks, are SMTP replies under the hood. Reading the code and text in a bounce message tells you whether the address is dead (550 5.1.1), the server wants you to try later (4xx), or your mail was refused for policy or reputation reasons (often 5.7.x).

Skrap.ai's mailbox check uses SMTP: it connects to the domain's mail server, asks about the recipient and reads the reply, without sending an email. That reply, combined with syntax, MX and catch-all checks, produces a Valid, Risky, Invalid or Unknown status. See it in the free email verifier.