Thanks for the clarification. I just received this "reject" notice when sending a test msg. So, if I understand correctly, it is non-deliverable from the outside world and can simply be used as the "From:" address in my MUA?
ckrzen@fedoraproject.org: host mx1.redhat.com[209.132.183.28] said: 550 5.1.1 ckrzen@fedoraproject.org: Recipient address rejected: User unknown in local recipient table (in reply to RCPT TO command)