Macs - Entourage - Split DNS - SBS2003 - Cisco 1841

Here's a question...

In this client's domain, there are a couple of servers.  Both SBS2003.  It is set up with an internal mydomain.local address.

The server has an external name - mail.mydomain.com.

The Macs have been set up with WebDAV and Entourage... and point to mail.mydomain.com... so on the SBS2003 DNS Server, mail.mydomain.com has to be defined as the 192.168.0.2 address of the server.  If it is not then the request for mail.mydomain.com is forwarded off to external DNS servers, which returns the address of the external interface of the Cisco 1841 router...

Clients on the outside can see mail.mydomain.com ... but they often open a VPN connection to the Cisco 1841 router... the router gives them a DNS of something.. it's own DNS proxy perhaps?

So the client has a network connection somewhere in the world... with DNS local to that ISP... they connect to the VPN and are given an IP address of 192.168.0.x ... and DNS is something...

but they can no longer connect to mail.mydomain.com ...

Is there an easy answer?   My only answer so far is to look up CCNA courses nearby... :)  The company who should look after this router want £NNN for a solution they can't guarantee will work... And it's not a client we want to mess around with... it has to work or not.