I have seen several questions and answers regarding this issue, however my situation is slightly different (I believe).
We have several emails in queue on our Exchange 2007 server that have the message:
451 4.4.0 Primary target IP address responded with: "571 Delivery not authorized, message refused"
I have checked and we are not on any blacklists, however we use Postini, so our MX records have four postini servers before our email server. I noticed that many folks were fixed by setting up reverse DNS entries via PTR records - however I think our Postini prevents me from pointing directly to my email server.
Is there something else I should be doing to facilitate delivery of these messages?