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<b>First and foremost:</b><br />
My Exchange environment consists of both Server 2003 and Server 2008 clusters running Exchange 2007 SP2. All of my users log in using Smart Cards. <br />
<br />
Some of my users are continually getting prompted for username and password when running Outlook 2007. The only users experiencing this are using Windows XP and have their mailbox hosted on one of my 2008 servers. When I move their mailbox to a 2003 server, they no longer get prompts for username/password. <br />
<br />
I've called Microsoft, but they won't help because they no longer support Windows XP. Any help is appreciated. Thanks<br />
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First and foremost:
My Exchange environment consists of both Server 2003 and Server 2008 clusters running Exchange 2007 SP2. All of my users log in using Smart Cards. 

Some of my users are continually getting prompted for username and password when running Outlook 2007. The only users experiencing this are using Windows XP and have their mailbox hosted on one of my 2008 servers. When I move their mailbox to a 2003 server, they no longer get prompts for username/password. 

I've called Microsoft, but they won't help because they no longer support Windows XP. Any help is appreciated. Thanks
 

Windows XP users getting prompted by Outlook for username/password.

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Windows XP is the sixth release of the NT series of operating systems, and was the first to be marketed in a variety of editions: XP Home and XP Professional, designed for business and power users. The advanced features in XP Professional are generally disabled in Home Edition, but are there and can be activated. There were two 64-bit editions, an embedded edition and a tablet edition.

Windows XP

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.