We have Exchange 2010 SP1. We had an issue a while back where some email domains were stuck in the Queue. I don't recall the reason, but I remember at that time reading somewhere to set-SendConnector ignorestartTTLS to True which resolved it. Just a couple of days ago started having domain stuck. The error is 451 4.4.0 DNS query failed.
The one thing I noticed is that on any of these domains if I run nslookup -q=mx domain name it fails. But the client says they are receiving mail from others and my other mail is working.
Any one have any ideas what could be wrong. Please help.