Question : How to setup DNS so that we can navigate to mail.our_domain.com

We have always had to use an internal address and an external address when using OWA from inside our company. I need to setup DNS so that we can use mail.our_domain.com to access owa from inside the company. This works fine for all external users but not internally. Internally we must use http://servername/owa. Our domain is currently domain.local.

Answer : How to setup DNS so that we can navigate to mail.our_domain.com

What you'd need to do is set up an internal DNS zone for our_domain.com.  Then you're manually updating all the our_domain.com assets that are on the Internet with their public-facing IP addresses, and internally-hosted ones with their internal addresses.  So you'd be maintaining two sets of DNS for our_domain.com.  Nothing wrong with that; lots of folks do it.  Sometimes is a pain, if you forget to update both when a server changes.

-tom