Not sure i understand what you mean by "external servers". If they have internet routable addresses your DNS forwarders would take care of your lookups. But i am guessing if that was the case you wouldnt be posting this question as it wouldnt be an issue. So by "external servers" you must mean a server outside of your domain but located within your LAN possible by VPN or frame connections. So I offer this solution for said issue. You need to setup a new or multiple new forward lookup zones and possibly reverse lookup zones if you wish. Example, if you want to connect to mail.notmydomain.com you would create a zone for notmydomain.com and add a host(A) record in that zone for mail with its IP. Do that for each unique domain and you should accomplish what you want. Also, dont forget to remove the enteries in the host files, your machines look at the host file first before turning to the DNS server for lookups.
Hope this helps,
RoadKi11