Many mail daemons will "accept" mail no matter who the recipient is. Internally, unmatched mail may be discarded or forwarded to a "catch-all" account, but all

Many mail daemons will "accept" mail no matter who the recipient is. Internally, unmatched mail may be discarded or forwarded to a "catch-all" account, but all the sender sees is "Recipient ok" I think this started as a response to SPAM bots that used to "RCPT TO:" random strings and save a list of valid address.