I have set up a new SMTP server using Windows Server 2012 R2 and the SMTP service (IIS 6). The server sits in our local domain but does nothing else for us.
For information (this is relevant) the server's DNS configuration is (in order):
The first 2 DNS servers are internal only - no internet access.
The problem I have is that with the DNS servers configured in this order no email will be delivered - the messages just sit in the queue. If I edit the order and place Google's DNS server first email delivery works every time - but I get untold numbers of local DNS related issues on the server over time.
With the DNS set up in the way I have listed above (i.e. Google last) my server will happily resolve any name (public or private) I ask it to.
It has unrestricted internet access.
This one has me scratching my head - so any advice will be gratefully received.