Internal vs External DNS Lookup's

I have a new Bell fibe router which apparently for "Security Reasons" does not support NAT reflection/Loopback.  In that I can no longer internally reach mail.mydomain.com which is a resolvable host NAT forwarded to one of my internal servers on mydomain.local.  To resolve this I have created a DNS entry on my internal DNS server to resolve mail.mydomain.com to the internal IP Address of mailserver.mydomain.local.  For the most part this solution works, however occassionally and seemingly randomly (mostly at initial WS startup) the mail.mydomain.com will internally resolve to the external IP.  NSlookup responds with correct IP Address of the internal host but an internal ping of mail.mydomain.com tries to ping via external IP address.  I have tried ipconfig /flushdns with same result.  This is a major nuisance as it causes Outlook to not be able to connect to our exchange server.

I have "patched" this for the time being by creating a host record on workstations pointing mail.mydomain.com to the internal IP, this solution is a bit of a wank, and obviously will not work for laptops that require access to mail.mydomain.com from both internal and external networks.

Is there a way to diagnose why it is resolving to external IP address and/or on my internal network to force the DNS to resolve to the internal IP address?

DNS search order from DHCP scope to clients is

192.168.135.15 (internal DNS server)
192.168.135.1 (bell fibe router)
8.8.8.8 (google)

Thanks
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BMardenAsked:
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Neil RussellTechnical Development LeadCommented:
You should ONLY have your internal DNS server on the clients. Then set up a forwarder on your internal DNS server to resolve unknow domains. This way your clients will never talk to anything except your internal DNS server and that knows the address of the internal IP
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StampelCommented:
Would it be possible to remove 8.8.8.8 from your DNS order scope ?
I guess this entry could be causing the problem and should be useless if your other DNS do the work
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BMardenAuthor Commented:
I could as a troubleshooting step, it was added because I noticed that occasionally the router was a bit slow doing DNS request forwards, also the router is forwarding DNS requests to same 8.8.8.8 any hoo, bit I will give it a shot.

Any other feedback?

Thanks
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StampelCommented:
It was my best guess i am confident but ...
Does nslookup for mail.mydomain.com respond the same for 192.168.135.15 and 192.168.135.1 dns ?
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BMardenAuthor Commented:
Thanks, should have thought of that
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