When sending emails to non local domains, the response should be relaying denied instead of Recipient address rejected: User unknown

for example:

Connected to mx1.domain.com.
Escape character is '^]'.
220 mail.domain.com ESMTP Welcome to MY ESMTP Server
helo r
250 mail.domain.com says HELO to 1.1.1.1:34137
mail from:xxx@yahoo.com
250 MAIL FROM accepted
rcpt to:xxx@gmail.com
554 5.1.2 Recipient address rejected: User unknown

See case #3081127