Sometimes you may want to be able to send  email from non existent email addresses on your own networks for notification purposes, you may get error “Sender address rejected: User unknown in virtual mailbox table”. In order to over come this issue, you have to disable this option : “smtpd_reject_unlisted_sender”

Uncomment the following line in /etc/postfix/main.cf

smtpd_reject_unlisted_sender = yes