For a Microsoft Outlook 2002 version of this article, see 287686.
This article describes why Microsoft Outlook requires the installation of Microsoft Outlook Express. Microsoft does not support the use of Outlook 2000 without Outlook Express installed.
Outlook shares files with Outlook Express. If you install Outlook with the Internet Mail Only (IMO) option, Outlook will not run without Outlook Express installed. If you install Outlook with the Corporate or Workgroup (CW) option, Outlook loses much of its functionality. Shared functionality between Outlook and Outlook Express includes the following key components:
Management of Internet Accounts
Use of SMIME
For additional information about issues you may encounter if Outlook Express is not installed, please see the following articles in the Microsoft Knowledge Base:
195879 OL2000: Installing and Using the Outlook Newsreader
212991OL2000: Error Message: "Outlook Requires Microsoft Outlook Express 4.01 or greater"
223473OL2000: Error Message "Out of memory. Close some windows or programs and try again."
224876OL2000: Error Mesage: An OLE Registration Error Occurred
For more information about selecting your default mail client, click the following article number to view the article in the Microsoft Knowledge Base:
196257 How to change your default mail client in Outlook