Exchange 2003 client setup username is case sensitive

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