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Read this, it talks about same issue as mentioned by you. <br />
<br />
<a href="http://social.technet.microsoft.com/Forums/en-AU/exchangesvrclients/thread/2bd0471f-751c-4f66-bf19-9abfd00d3cba" rel="ugc nofollow">http://social.technet.microsoft.com/Forums/en-AU/exchangesvrclients/thread/2bd0471f-751c-4f66-bf19-9abfd00d3cba</a>
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are you using basic or ntlm under the outlook proxy settings? &nbsp;and, is it the same on the xp and 7 machines?
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Thanks amitkulshrestha but that link suggestions I already followed, actually there is no stated solution on that link.<br />
<br />
madhatter5501: Thanks for your comments, I am doing some testing regarding to this.<br />
<br />
I´ll keep you posted. 
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I found the answer on an MS forum.
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<div class="content wysiwyg-content">
Hi there!<br />
<br />
I am having a lot of trouble to understand what the problem is. I´ve migrated a domain from Windows 2003 to Windows 2008 R2, witihn the migration we also migrated from Exchange 2007 to Exchange 2010.<br />
<br />
All computers using Windows 7 dont have any problem using Outlook 2007 or 2010, but users using Windows XP Pro have problems to connect, and I always get a password prompt.<br />
<br />
The only way I´ve been able to connect this Windows XP Pro´s Outlooks is by disabling MSSTD line on the Exchange &gt;&nbsp;Connections tab.<br />
<br />
The thing is that disabling this parameter works for one session or so and for some reason it gets activated again and fails to connect.<br />
<br />
Any ideas why is this happening and how to solve it? <br />
<br />
Thanks a lot!
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Hi there!

I am having a lot of trouble to understand what the problem is. I´ve migrated a domain from Windows 2003 to Windows 2008 R2, witihn the migration we also migrated from Exchange 2007 to Exchange 2010.

All computers using Windows 7 dont have any problem using Outlook 2007 or 2010, but users using Windows XP Pro have problems to connect, and I always get a password prompt.

The only way I´ve been able to connect this Windows XP Pro´s Outlooks is by disabling MSSTD line on the Exchange > Connections tab.

The thing is that disabling this parameter works for one session or so and for some reason it gets activated again and fails to connect.

Any ideas why is this happening and how to solve it? 

Thanks a lot!

Outlooks in Windows XP wont connect to Exchange 2010

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 Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

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.