What is RCPT TO?
Also called: SMTP mailbox probe, recipient verification
RCPT TO is the SMTP command that names the 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 reply code, and disconnect without ever sending a message.
This is what separates real verification from a syntax check. A 250 reply means the server will accept mail for that address; a 550 usually means the mailbox does not exist. Nothing is delivered, no message body is sent, and the connection closes after the reply.
It is also why verification needs outbound port 25. Many cloud providers block it by default, and a verifier running on a blocked host cannot tell a real mailbox from a made-up one. Every address on a live domain then comes back unconfirmed, and the honest thing is to report it as risky.
Servers vary in how much they will say. Some accept everything (catch-all), some greylist unfamiliar senders, and some, including several large consumer providers, answer the same way for every address to defeat address harvesting. Each of those is an inconclusive result, not a valid one.
Example
RCPT TO:<[email protected]> → 250 2.1.5 OK (accepted) or 550 5.1.1 The email account that you tried to reach does not exist (rejected).How SimpleVerifier handles it
A 250 or 251 reply is valid · Mailbox exists; 550, 551, 553 or 554 is invalid · Mailbox not found; a temporary 4xx reply is risky · Server would not confirm. Microsoft consumer domains such as Outlook.com, which are not probed while Microsoft refuses checks from our server's network, return risky · Provider hides mailbox status.
Go deeper
Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms