Exchange 2003 setting up Outlook 2003 for new user

We have an AD environment running Exchange 2003. Sometime in the past few months, when a new user is created and we setup their Outlook 2003 client we must enter the user name in the setup screen exactly as it was created. I mean it is now case sensitive. Their logon name is not case sensitive, just when setting up outlook.
Is there a setting in Exchange to turn this on and off? Did some recent patch make this happen?
Thanks,
Michael