In SMTP connection, after passing wrong email address to RCPT command, we are receiving "550 Unknown user" error from one email server and receiving "250 OK" response from another email server. In which scenarios are we supposed to receive "550 Unknown user"?
Also, after receiving "550 Unknown user" response, Rebex is sending RSET command. Is it valid to send reset command on receiving 550 error? Do we have any RFC references in detail answering these questions?
550 Unknown user