Outlook must be enforced as the default email, calendar, and contacts program.

Outlook is made the default program for E-mail, contacts, and calendar services when it is installed, although users can designate other programs as the default programs for these services. If another application is used to provide these services and your organization does not ensure the security of that application, it could be exploited to gain access to sensitive information or launch other malicious attacks. If your organization has policies that govern the use of personal information management software, allowing users to change the default configuration could enable them to violate such policies.