When a user wants to access Microsoft Dynamics CRM records within the Microsoft Dynamics CRM for Microsoft Office Outlook client, they may encounter intermittent prompts for credentials.
The issue occurs because of a problem in Internet Explorer problem that causes session cookies to be cleared out and requires users to sign in again.
The Internet Explorer team has addressed this issue as a hotfix, which can be found in the following Microsoft Knowledge Base article:
2926827 Web browser control hosting application may lose session information unexpectedly
This issue affects both Internet Explorer 10 and Internet Explorer 11.