I want to set up our Mac OS X Lion DNS server to host a specific domain internally, but redirect all other queries to our providers dns servers. How can I achieve this? I have read the docs but couldn't find anything about it.
This is actually the default behaviour :-)
I strongly recommend you read up more from the following excellent resource, which is considered by many to be the definite description of DNS best practice in Mac OS X Server: Hoffman Labs: "DNS Tips: Establishing a DNS Server on Mac OS X Server 10.7 or 10.6"
If you want to specify what DNS servers are consulted for lookups in domains for which your server is not authorative, you can specify them as forwarders in Server Admin under the DNS service's Settings. Have a look at Network services ▸ Domain Name System (DNS) ▸ About DNS ▸ Set up namespace behind a NAT gateway in Apple's Lion Server help, as well as Network services ▸ Domain Name System (DNS) ▸ Manage DNS zones ▸ Set forwarder IP addresses
To clarify further: by default behaviour, I mean that I've found, in practice, that configuring forwarders, though best practice, is optional, because if you don't do it, when a Mac OS X Server running DNS receives a query for which it has no corresponding primary or secondary zone, it will automatically query whatever DNS servers it uses for its own Internet connection and return that information. So basically, you could just set up and maintain the zone for the domain you want to host internally and the rest should happen automatically.
(2012-01-30 17:30 EET — Edits to fix grammar and expand and clarify initial statement.)