A server's mail.log says [I have changed a couple of details to
"invalid" below]:

Feb 14 14:43:27 mwuml007 postfix/smtpd[25708]: NOQUEUE: reject: RCPT
from unknown[216.220.221.222]: 450 Client host rejected: cannot find
your hostname, [216.220.221.222]; from=
to= proto=ESMTP helo=<mfe1.prod2.dngr.net>

The MTA at invalid.co.uk has rejecetd the message from
 because the sending MTA claims to be
<mfe1.prod2.dngr.net> whereas its address in fact turns out to be
something else - is that correct?

Thanks,

Daniele
-- 
<http://vurt.org>