I'm using suse 7.2 on my machine and use fetchmail to get my remote email, mostly I get this error.
fetchmail: SMTP< 451 4.1.8 <owner-postfix-users@postfix.org>... Domain of sender address owner-postfix-users@postfix.org does not resolve
But I can browse to all domains from the same machine, ssh works, and so forth...it just doesn't seem to work with fetchmail and sometimes nslookup gives the unknown domain error.
Very strange...
Likely it's the DNS resolver problem on the other end, not yours. postfix has such spam blocking feature for a while.