Exchange Server Name uses local full Netbios name

We have exchange server 2003 setup in our office and we have everything working great, except of the laptop users.  The exchange server address should be, but when we put this in and select 'Check Name', it automatically reverts to  Not a problem except for the laptop users.  Once they leave the office, they get password errors and if we change it to, it stays like that until they return to the office, and then it reverts again.  I'm sure I'm missing something in DNS or Exchange Server settings...any help or direction would be greatly appreciated.
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This may be a firewall issue.  Once they leave the office, ordinarily they would not have full access to the Exchange server in the same way that they do when they are in the office unless they are using a VPN. Are your mobile users using a VPN when they leave the office?
If you are using VPN or outlook over RPC the server name is the internal server name.
you then configure rpc with the OWA address.

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