We're getting undeliverable messages with increasing frequency. I've done quite a bit of research and it seems that the DNS is failing, prompting this error message:
#554 5.4.4 SMTPSEND.DNS.NonExistentDomain; nonexistent domain ##
When I do an nslookup on the domain from the Exchange server, it returns "*** dc1.mydomain.local can't find externaldomain.com: Non-existent domain". However, when I do the nslookup on the dc1 server, it returns the non-authoratative answer.
The Exchange server and domain controller are both running 3rd-party anti-malware, but we've been using it for a long time and this issue is just coming up. It's a new issue and it seems to be getting worse and worse by the minute today.