Generally, you don't need/want port forwarding inside your LAN. If you hit mail.mydomain.com, it will still resolve to your WAN address and port forward to the right machine inside your LAN. Or, you create a DNS enty on the router so that mail.mydomain.com resolves to the actual LAN address of your mail server on your LAN. That is why the router attempts to act as it's own DNS server - so it can do special handling of names inside your LAN.
In my case, my router's LAN address is 192.168.1.1. When all the devices on my LAN use DHCP to get an IP address, they are all told that 192.168.1.1 is the DNS server. When these devices attempt to do a name lookup, it hits my router and my router decides if it wants to answer the question locally or pass the request along to the real DNS external server. When you add port 53 to your port forward list, this all breaks. Now, every device on my LAN hits 192.168.1.1 expecting to find a DNS, but instead is forwarded to my Xbox (which is powered off) and the request eventaully times out.