We have a problem with our transport server sending e-mail to other exchange servers in different sites within our forest.
The transport server which is trying to send the emails is a HUB/CAS exchange 2010 server
When trying to send e-mails to users in other sites the exchange server attempts to route the emails as expected vias the SMTP Relay to Remote Acrive directory site (Inter Organisational) however we continue to receive the error in the screen shot (451 4.4.0 DNS query failed. SMTPSEND.DNS.NonExistendDo
All the server names are in the DNS server correctly and can all be pinged and reached from the transport server without any problems.
Lastly if we ignore these errors eventually after a period of an hour or so the mails do get delivered, however as you can imagine this is not ideal.