OWA Forms Based Authentication settings

The Settings for Forms Based Authentication can be set for each sub directory of Exchange
Default Sub Directories depend on your migration path

/exchweb
/exchange
/Public
/OWA

OWA is the new subdirectory used for Exchange 2007 outlook web-access, and should be the shortcut you give all your clients.
Right Click on the Sub-directory you would like to Authentication for then Click Properties, then click Authentication Tab.

Set your prefered Authentication Method

DOMAIN\Username: requires that you enter the domain first
Users Prinicipal Name is their Logon (Email) Address, The User@DomainName format
Finally, Username only means you only need to specify the username, select the Domain Name with the Browse button, and this is the domain name you will authenticate too.

 This will require you to restart IIS on your Exchange Server for the changes to take effect.