RCPT TO
The SMTP command that declares each envelope recipient, sent once per recipient after MAIL FROM.
`RCPT TO` is the SMTP command that declares one envelope recipient. The client sends it once per recipient after `MAIL FROM`, and the server accepts or rejects each one independently (250 OK, 550 mailbox unknown, 451 try later). This per-recipient handling is why a single message can succeed for some addresses and fail for others on the same delivery attempt. Modern anti-spam systems often delay the final accept/reject decision until after DATA, but the RCPT TO step is still where address-validation, recipient-throttling, and recipient-specific policy are enforced.