If you are experiencing an issue where Outlook 2013 keeps prompting for Credentials and you are connecting to Hosted Exchange on Office 365 here is one simple fix which may resolve the problem. Try deleting the credentials for Office 365 in the Windows Credential Manager.
Open the Control Panel and Click on Credential Manager
Find your Office 365 credentials as per the diagram below and choose Remove from Vault
Open Outlook 2013 again, enter your credentials and tick the box to remember the password
After connecting, close outlook and open it again
Hopefully this tip resolves the issue of Outlook 2013 prompting for Credentials when connecting to Office 365. We’ve found that it resolves the problem in about 90% of cases.