outlook 2003 default email

I have four XP pro machines with outlook 2003 client installed and using the exchange server 2003. Also each of the clients has a third party email account eg. mail.xxxx.com setup in addition to Exchange server. My question is how do you get the default to stay set. The problem I have is I can set the default to the third party account, but, when the user logs off and logs back on the next day the default has changed back to the Exchange server. I'm sure there's a setting on the server, but I don't know where to look. Any help would be greatly appreciated.

Ron