Outlook asking for password

Hi
I have an Exchange 2003 server running with 6 remote offices connected to it with a permanent VPN. Each office has a different domain name.
Every remote user is prompted for a username and password every time they open up Outlook.
I have added the username and password in the Manage Passwords section of Control Panel in the format
Server: server
User name: domain.local\user
Password: password.

It keeps asking even when I tick the Remember password box.

Anyone know how to force Windows to remember the password?
Thanks in advance.