[Blind-sysadmins] setting username format in Microsoft exchange?

Hi Listers, Sorry for the somewhat vague subject. To those who have setup an on-premmiesis exchange server, How do you go about having a user accessing mails via outlook logon just using their email address and password? As it currently is, I have to connect using: Domain\username followed by the password. I'd like to authenticate with just the email address and password. I've googled, but can only find a way to change it for OWA and not outlook or any other client. Your help would be much appreciated. Kind Regards