Outlook Prompting for Credentials

There are many reasons why outlook might prompt for credentials. However, one of the reasons that I often forget is when outlook tries to access an internal address or server via a web proxy, i.e. it does not know to bypass the web proxy when looking for the address or server in question.

Ensure all servers (including aliases) that Outlook should connect with internally are excluded in your web proxy configuration.

For other potential causes/fixes,  Ilan Lanz (Ilantz) has created a great troubleshooting list here.

References:
http://ilantz.com/2011/02/08/authentication-pop-ups-and-annoyances-with-exchange-2007-2010-and-outlook-anywhere/