DNS resolves servers external IP instead of internal IP
It sounds like what you're trying to do is called Split Horizon DNS, and it can be quite tricky. That said, it sounds like you have quite a bit of it already set up but that the internal DNS is not authoritative for your domain.

If you have not already done so, you need to modify your internal DNS to create a primary zone for example.com. Then, when you query for your domain, the DNS should not look outside for an address.