(change 10.0 to 9.0 for Outlook 2000 SP3,11.0 for Outlook 2003, 12.0 for Outlook 2007)
Under that key, add a new string value named Level1Remove.
For the value for Level1Remove, enter a semicolon-delimited list of file extensions. For example, entering this:
would unblock Microsoft Access files and Internet shortcuts. Note that the use of a leading dot was not previously required, however, new security patches may require it. If you are using "mdb;url" format and extensions are blocked, add a dot to each extension.
Note also that there is not a space between extensions.
If you are using this registry entry, a glance at Help>About Microsoft Outlook will show Security Mode: User Controlled above the license information.
After applying this registry fix or using one of the above tools, the user still has to save the attached file to a system drive before opening it. In effect, the fix rolls the attachment behavior back to Outlook 2000 SR-1, with its included Attachment Security Fix.
An end-user cannot bypass this "save to disk" behavior and open the file directly from the mail message, though an Exchange administrator can.