When sending emails to non local domains, the response should be relaying denied instead of Recipient address rejected: User unknown
Connected to mx1.domain.com.
Escape character is '^]'.
220 mail.domain.com ESMTP Welcome to MY ESMTP Server
250 mail.domain.com says HELO to 188.8.131.52:34137
250 MAIL FROM accepted
554 5.1.2 Recipient address rejected: User unknown
See case #3081127