my customer ran into a problem that his exchange server just didn’t receive any mail no more or couldn’t send mail also.
when i checked the mail queue i found the following error:
451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.NonExistentDomain; nonexistent domain
after checking everything from connectors and exchange settings, I started to check just everything, and the first steps i always do is: are the TCP/ip config correct on your network card(s).
turns out the DNS 2 entry was set to external dns 188.8.131.52
once you remove this DNS entry and only use the internal DNS server everything started working again.