This is usually caused when a user has previously set their Outlook client to store the password and the password and since been changed. This results in Outlook constantly trying to login using the old stored credentials.
- Select File, then Account Settings > Accounts Settings
- Now select the Exchange account that Outlook is prompting the login credentials for and click Change
- Click on More Settings
- Select the Security tab and check the box next to Always prompt for logon credentials
- Click Ok, then on the Accounts Settings window
- Click on Next then Finish.
From this point, Outlook should only prompt you on the initial launch of the program for your login information.