Outlook is not retaining the credentials due to your Outlook profile been corrupted.
Create a new Outlook profile.
Also, Outlook shouldn't be prompting for credentials.
1.Make sure you are not locked out of the system.
2.Password is not expired.
3.The profile security is not set to prompt for logon credentials.
-Tools>Account Settings>Change>More settings>Security>unchek Always prompt for credentials>Apply and restart Outlook.
4.MS Exchange could be connecting via HTTP.
-Tools>Account Settings>Change>More settings>Connection>Under the Outlook Anywhere section, make sure 'Connect to Microsoft Exchange using HTTP' is unchecked.