I have a similar issue and this is what I did:
I have an outside webhosting company hosting
www.xxxxxx.com, they are forwarding all email to my sme server at yyyyyy.com. I created a virtual host on my sme server called xxxxxx.com. Under hostnames and addresses (in server-mgr) I modified all the the hostname entries for remote location and pointed them to the IP address of the 3rd party WEBHOSTING server.
The results:
When i type in xxxxxx.com it simply goes to my SME box, when I type in ftp, www, or mail.xxxxxx.com, it goes to the remote 3rd party webhosting server. Note, this is only when I access this from within the LAN, all requests made from outside my LAN are treating by the normal DNS servers at the ISP from which I am using.
Hope this helps....
SloopJohnB