Hello,I am writing for help with an Exchange 2007 routing issue. Suddenly mail to one of our business partners is being queued on our server with the error "451 4.4.0 DNS query failed". We have always been able to send email to this domain until 2 days ago when I received a mail delivery delayed notice from my server. I can receive mail from this domain fine. All other outgoing mail is fine, this is only happening in 1 queue.I've tried several troubleshooting steps: I verifed the external and internal DNS lookups for the mailserver are and have been set to "all available NICs". On the server I ran nslookup with type=mx and I always get a response from the domain in question. I verified with the recipients that no outages or new software has been installed on their end. I verifed that Ironport is not blacklisting my domain.Can anyone help explain this? Here is my setup:Exchange 2007 STD hub transport and CAS role server (no SP level)2003 STD server R2 64bit OSThank you,Rob

Hi, i had the same problem. In the DNS property of te Exchange Server NIC, set a public secondary DNS server. In my case this solved the problem. regards Luca

Do you know where I can find a list of public DNS servers? Is there a living list that will contain changes?Thanks!

you should use the name servers of your internet provider.