Hey Max Bee,
First, start with creating a new account on your XP machine. Name it MCX2 or something. Logon to Windows with this new account so the users profile is created. logoff, and login as an adminitrator. Now replace that newly created profile with the profile of your actual extender user witch is usualy MCX1. Don't just copy it with the explorer, use the "User Profiles" option from the properties of the "My Computer" icon on your desktop. Don't forget to hand out rights/priviledges to the MCX2 user!
Now logon with the MCX2 account and setup your outlook account, make sure it uses the Exchange mailbox you wanna use on your extender. Start MyOutlook and double check everything works like it should. If it don't work in a regular windows session, it surely won't work on the extneder!!!
Now logoff and logon as an Administrator again, and repeat the profile copy process, now copying the MCX2 profile to the MCX1 profile (again, don't forget to set the priviledges) and make MCX1 also a member of *all* the groups MCX2 is a member of. Now start your xbox and have fun
If everything works you can safely remove the MCX2 account.
If it doesn't work, please explain witch part doesn't work...
Good luck, and have fun.
Cheers
Cacti