Unable to set internal DNS

Question

I have an OS X 10.6.8 Server with DNS and Mail running on it.
The internal domain does not match the external domain.
Users can send and receive IMAP email on iPhones, iPads and laptops whilst outside the network using 'mail.mydomain.com' with corect account details.
A and PTR lookups resolve correctly using the internal domain on the server and the external domain on the internet.
webmail.mydomain.com also works perfectly outside the network but is unreachable using https://webmail.mydomain.com:443 internally - and it should.
There is a ALIAS set up in the servers DNS that points webmail.mydomain.com (external) to server.mydomain.com (internal).
I am using a ZyXel P-660HN-F1Z Router with the firewall turned off and all the port forwarding correct... otherwise the external mail wouldn't work!
Previously we used a BT 2wire Gateway that didn't do anything clever - but all the mail worked internally and externally.
Is it my router config, or the DNS on the server screwed?
Would really love some help.
Thanks
Simon

Answer 1

There's not enough information to be sure of your configuration.
it would seem appropriate to set up your external domain - I'll refer to that external domain as example.com as your mydomain.com is a real and registered domain - as the MX record for your internal domain which I'll refer to as example.net. 
With this configuration, there would be no internal definitions (A machine records or CNAME alias records for any of the example.com hosts in your internal DNS services.
If you're using the same example.com domain both within your local network and a second example.com implementation on a second and separate and external DNS server configuration, then you'll need to reference all the hosts directly in both places; in your internal DNS services configuration, and you'll need to replicate all definitions of all hosts in your external DNS services configuration.
See if your internal network can ping (if that's enabled) or telnet into port 25 or such using your external domain name, as that'll tell you if your router is smart enough to pass packages destined for your public static IP address back into your network.
Your internal hosts should all references ONLY your local DNS server on your LAN, and NO other DNS servers.  Again, your internal hosts should reference ONLY your internal DNS server, and should not also reference your ISP DNS or other external DNS servers.
There's a list of internal DNS services setup information here, and there are also links from that article to articles around setting up external DNS services; DNS inside your firewall, and DNS outside your firewall.