Hello,
I get lots of those errors in my postfix log:

Jul 3 18:09:16 server postfix/smtpd[2840]: NOQUEUE: reject: RCPT from
tupac2.dyne.org[178.62.188.7]: 450 4.7.1 <tupac2.dyne.org>: Helo command
rejected: Host not found; from=<dng-bounces@???>
to=<jf@???> proto=ESMTP helo=<tupac2.dyne.org>

Is there some configuration problem with the list mail server?

Jochen