*) Could you please paste original log in Postfix log file (/var/log/mail.log)? Not just the line contains error message, but all related logs before and after it.
*) Do you have correct DNS server(s) in /etc/resolv.conf and /var/spool/postfix/etc/resolv.conf?
*) Do you have correct hostname in /etc/hosts and /var/spool/postfix/etc/hosts?