What is a rcpt to?
Also called: SMTP mailbox probe, recipient verification
RCPT TO is the SMTP command that names the intended recipient of a message. Verification tools use it to test whether a mailbox exists: they connect to the recipient's mail server, issue RCPT TO, read the response code, and disconnect without ever sending a message.
This is what separates real verification from a syntax check. A 250 response means the server accepts mail for that address; a 550 means the mailbox does not exist. Nothing is delivered, no message body is transmitted, and the connection closes after the response.
It is also why verification needs outbound port 25. Most cloud providers block it by default — DigitalOcean, Vultr and AWS routinely refuse, and a verifier running on a blocked host cannot distinguish a real mailbox from a fabricated one. Every address on a live domain comes back unconfirmed, which the honest response is to report as risky.
Servers vary in how much they will tell you. Some accept everything (catch-all), some greylist unfamiliar senders, and some deliberately answer 250 to all probes to defeat address harvesting. Each of those is an inconclusive result, not a valid one.
Last reviewed 2026-09-11