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Is Autodiscover working properly in your Exchange environment? &nbsp;Perhaps a certificate issue? &nbsp;I would start with these.
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Is Autodiscover working properly in your Exchange environment?  Perhaps a certificate issue?  I would start with these.

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Do you see anything amiss in the event logs on your Exchange server? &nbsp;And check here, first reply post, see if this is similar to your issue.<br />
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<a href="http://serverfault.com/questions/42902/outlook-2003-is-prompting-my-active-directory-users-for-their-logon-credentials" rel="ugc">http://serverfault.com/questions/42902/outlook-2003-is-prompting-my-active-directory-users-for-their-logon-credentials</a>
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Do you see anything amiss in the event logs on your Exchange server?  And check here, first reply post, see if this is similar to your issue.

http://serverfault.com/questions/42902/outlook-2003-is-prompting-my-active-directory-users-for-their-logon-credentials [http://serverfault.com/questions/42902/outlook-2003-is-prompting-my-active-directory-users-for-their-logon-credentials]

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Windows 2000 active directory, exchange server 2003, outlook 2003, with a mix of XP and Win7 in a mature environment has been working for years without any issue. &nbsp;Out of 100+ users, a random subset of users, some on XP, some on Win7, will get prompted repeatedly for a password to log into Outlook. &nbsp;When the correct password is keyed in, it just prompts again, so they effectively cannot then get in to Outlook at all. &nbsp;Sometimes a close and reopen fixes the problem, sometimes a reboot does, but not consistently, and the problem always reappears eventually, usually within 24 hours. &nbsp;All other users on the same system are fine. &nbsp;<br />
Can't seem to find any commonality among the affected users, except that they all use Outlook. &nbsp;Flat IP network.
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Windows 2000 active directory, exchange server 2003, outlook 2003, with a mix of XP and Win7 in a mature environment has been working for years without any issue.  Out of 100+ users, a random subset of users, some on XP, some on Win7, will get prompted repeatedly for a password to log into Outlook.  When the correct password is keyed in, it just prompts again, so they effectively cannot then get in to Outlook at all.  Sometimes a close and reopen fixes the problem, sometimes a reboot does, but not consistently, and the problem always reappears eventually, usually within 24 hours.  All other users on the same system are fine.  
Can't seem to find any commonality among the affected users, except that they all use Outlook.  Flat IP network.

Outlook 2003 random users prompted for password

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

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

An email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email. A web application that provides message management, composition, and reception functions may internally act as an email client; as a whole, it is commonly referred to as webmail. Likewise, email client may be referred to a piece of computer hardware or software whose primary or most visible role is to work as an email client. Email clients can also have other systems, like calendars, notes and contact managers.