You are prompted to enter your user name and password when you connect to Office 365 resources using a rich-client application

This may occur if the computer does not meet the system requirements for Office 365. 

Make sure that the computer meets the system requirements and that the latest updates are installed. Additionally, the following items should be installed on the computer:

• Microsoft Online Services ID Sign In Assistant (installed by Office 365 Desktop Setup)
• Operating system service packs and updates
• Microsoft Office service packs and updates

For more information about the system requirements for Office 365, visit the following Microsoft websites:

• Office 365 for enterprises: http://onlinehelp.microsoft.com/en-us/Office365-enterprises/ff652534.aspx
• Office 365 for professionals and small businesses: http://onlinehelp.microsoft.com/en-us/office365-smallbusinesses/ff652564.aspx

By design, Microsoft Outlook prompts users for credentials every time that Outlook is opened or every 8 hours, whether user is leveraging federated credentials or not.

If the computer does meet system requirements and federated users are prompted for credentials, perform Active Directory Federation Services (AD FS) troubleshooting for authenticating to Microsoft Online Services.

For information about Office 365-supported client applications on a Macintosh computer, see the following article:

2461500: Cannot access Office 365 resources by using a Macintosh computer