I was working on this annoying issue few days back, the mails sent to few domains are getting stuck in the queues on the Hub Transport server. The error message on the queue states 451 4.4.0 DNS query failed.
Well, it looked liked from here that there might be some issue with the DNS, ran few nslookup tests from the HUB servers itself, and surprisingly the domain and its MX records were resolving fine and i was able to connect to them absolutely fine.
More interestingly, if you hardcode the entry of the MX Record for that domain in the hosts file, the mails went through straight away.
I researched around few forum posts stated below, it seem like that this is a Bug with Exchange Server 2007 on Windows Server 2008, some issue related to IPv6.