Definition
RCPT TO: The SMTP command that names one recipient of a message (RFC 5321). The server reply is what mailbox verification reads: 250 accepts the address, 550 with 5.1.1 rejects it as unknown, and a 4xx defers the decision.
How to read RCPT TO replies
A 250 reply, usually with 2.1.5, means the server accepts the recipient. A 4xx reply means not now: greylisting, a rate limit, a full mailbox or too many recipients in one session.
A 5xx reply is a permanent no, and the enhanced code says why. 5.1.1 means the mailbox does not exist, 5.2.1 a disabled mailbox, and 5.7.1 a refusal on policy grounds.
Why a 250 reply is not always proof
Catch-all servers return 250 for every address, real or invented. Security gateways often do the same at the edge and check mailboxes only after the message arrives.
The way to tell is to ask for a random address in the same session. If the server accepts that too, the first 250 carried no information about the mailbox.
Why a 550 reply is not always a missing mailbox
550 is also used for rejections that have nothing to do with the recipient: the sending IP is on a blocklist, the sender failed authentication or the server simply does not like the connection.
The enhanced code and the reply text separate the two. Treating a policy block as an invalid address deletes good contacts from your list.
C: MAIL FROM:<probe@verify.example.net>
S: 250 2.1.0 Sender OK
C: RCPT TO:<ana@example.com>
S: 250 2.1.5 Recipient OK
C: RCPT TO:<nobody.here@example.com>
S: 550 5.1.1 User unknown
C: RCPT TO:<ben@example.com>
S: 550 5.7.1 Client host 203.0.113.7 rejected by local policyHow RocketVerifier handles it
Our engine sends RCPT TO from a dedicated verification domain and disconnects before DATA. When the real address gets a 250, it asks for 3 random addresses at the same domain in the same session, and if 2 of 3 are accepted the domain is treated as catch-all instead of the address being marked deliverable. A rejection about the sender, such as 550 5.7.1, is reported as blocked (unknown), and a bare 550 without a clear user-unknown meaning is treated as ambiguous rather than undeliverable.