If you're using Outlook Web Access with Exchange Server 5.0 or higher and Windows 2000 Server, then you have to let your users know what to expect when they first try to log on to the server over the Web.
Users will get a different authentication dialog box depending on the version of the browser they are using. Some of these dialog boxes have three fields for entering information: username, password, and the domain. So a user needs to know the name of the domain in which he or she authenticates, which could be the first problem a user encounters.
Assuming that bit of knowledge has been disseminated, the three-field dialog box is pretty clear for users, because the fields are clearly labeled. So Jenny Jones, for example, would enter "jjones" for her username, <password>, for her password, and, the domain name (accounting, for example). It's simple, straightforward, and to the point.
But the two-field box is more obscure. This dialog asks simply for a username and a password. In this case, the Exchange server still wants the same information before it will allow a user access to his mailbox. But in the username space, it's looking for the domain name first, followed by a backslash and then the username. So here the user would enter, for example, accountingjjones. She'd enter her password in the field identified for password as in the three-field dialog box already described.
Microsoft says, in a document titled "Troubleshooting Outlook Web Access in Microsoft Exchange 2000 Server" that the appearance of the two-field or three-field logon box is a function of the browser version. However, I have seen the three-field box in Windows 2000 with Internet Explorer 6.0, and the two-field box in Windows XP with IE 6.0. The best thing to do is make sure your users are aware that they could get either one, depending on whether they use OWA from their own computer or someone else's (from an Internet cafÉ, for example).
Note that when you're using basic authentication, which is the simplest of the authentication schemes available, users have to authenticate with their domain, username and password each time they attempt to use Outlook Web Access.