I have 12 DCs all running DNS. My mail gateway is using one in particular for DNS and lately we've been getting a few of these
The following message to <email@example.com> was undeliverable.
The reason for the problem:
5.4.7 - Delivery expired (message too old) 'DNS Soft Error looking up jpmchase.com (MX) while asking 192.168.75.x. Error was: ServFail'
so i tested looking up that MX record from that DC and it times out. When I look it up from all the other DCs in the domain it finds it fine. Also when I look up just the A record for JPMchase.com it finds it fine. The only difference in configuration is that this DC has forwarders on (pointed to our ISPs DNS servers). I took them off to see if that would make a difference but it still fails. I've run dcdiag and netdiag and both come back clean.