1. Run Regedit, and go to this key:

   HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security (change 10.0 to 9.0 for Outlook 2000 SP3 or to 11.0 for Outlook 2003, 12.0 for Outlook 2007 )

2. Under that key, add a new string value named Level1Remove.
3. For the value for Level1Remove, enter a semicolon-delimited list of file extensions. For example, entering this:

   .mdb;.url

   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. See OL2002 You Cannot Open Attachments for more information on this registry entry.

Operating System: Genuine Windows XP (Service Pack 2 or Service Pack 3); Windows Vista (Gold, Service Pack 1, or Service Pack 2); Windows 7