Hmmm.. cannot find your PM, I must be having a moment!
Anyway, I'll try and keep it short.
I have a couple of servers on my LAN, I also have a static WAN IP, well it's a Sky virtual static in that it's a fixed DHCP address but that's an aside.
I won't post my real domain name as I don't really want lots of attempts to hack my system, so I'll use the name foobar.com.
So, the global DNS servers know of the address lmail.foobar.com, any device attempting to connect to that will get my WAN IP address. My router then forwards the relevent ports to the correct server.
Now, imagine I am sat at home, with my iPhone/Android whatever and they are now connected to the local LAN and not EE/Vodafone or an external WiFi network and I want to collect mail. The mail clients are told to goto lmail.foobar.com to get the mail, sadly they are still going to get the WAN IP because the D7000 is not equipped with a LAN DNS resolver that knows that lmail.foobar.com is actually sat at 192.168.1.30 and not at 88.123.453.34 ( not my real IP!), so it fails. The router itself cannot have the domain foobar.com becuase that is hosted elswhere, it does not need to though, all it needs to know is that the server lmail.foobar.com lives on its LAN, not outside of it.
Yes, I could set the fixed PC's to point to the local address but that won't work with the portable devices.
On my Vigor I could set up LAN DNS names so it would check them first before checking the global DNS servers, even my old ASUS running 3rd party firmware could do it.
Have a shone a little light on it?