This article describes why Outlook requires Microsoft Internet Explorer, version 4.01 or later.

Many of the Outlook features require the presence of Internet Explorer. Outlook is integrated with Internet Explorer to provide such features as:

•	Using digital certificates for sending digital signatures, and sending and receiving encrypted messages
•	Using HTML formatted e-mail rendering and printing
•	Using security zones to set warning levels for received mail containing controls or applets
•	Setting and rendering Web View (Home Page view) of Outlook folders
•	Setting offline synchronization options for home pages up to three levels deep
•	Using Internet Explorer Favorites in Outlook
•	Using Web Discussions

For these reasons, if you decide to not install or upgrade to Internet Explorer 4.01 or greater, Outlook will not install. Outlook lacks the necessary program files to work without Internet Explorer.

For additional information about Outlook setup and Internet Explorer, please see the following article in the Microsoft Knowledge Base: