Outlook 2003, Outlook 2002 and Outlook 2000 SP3 (but not Outlook 98 or earlier Outlook 2000 versions) allow the user to use a registry key to open up access to blocked attachments. (Always make a backup before editing the registry.) To use this key:
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)
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. See OL2002 You Cannot Open Attachments for more information on this registry entry.
To force users to save *.zip files to the hard drive before opening, add .zip to the extensions step 3. See How to configure Outlook to block additional attachment file name extensions for more information.
If you prefer not to edit the registry directly, you can use one of these tools to make the change; not all support both Outlook 2002 and 2000:
Attachment Options COM add-in by Outlook MVP Ken Slovak that adds an Attachment Security Options page to the Tools | Options dialog in Outlook 2002 to allow you to manage which file attachments are blocked. Only for Outlook 2000 SP3, Outlook 2002, and Outlook 2003. For Also provides a user interface for setting two other Outlook options -- Read as Plain Text (Outlook 2002, SP-1 or later) and Minimize to System Tray. Available in English and German. Shareware.
Attachments Zip Compressor Automatically compresses outgoing attachments into .zip files. Can extract files from incoming .zip attachments or compress attachments in an incoming message. Can handle incoming files that Outlook blocks and put them into .zip files.
DetachOL Standalone utility to change the list of blocked file attachments according to the level of risk you're willing to take. Free.
Outlook Permissions Add-in Add-in for adjusting which attachments you can open under Outlook's increased security, for Outlook 2003, 2002 and Outlook 2000 SP3 or later. Free
TweakOL Modifies the Windows registry to change the list of blocked file attachments according to the level of risk you're willing to take, always show the Bcc field, minimize Outlook 2002 to the system tray, and disable or enable MSN Messenger. Free. Outlook 2002 only.
Xenos Outlook Security Extension Utility to manage file attachment blocking in Outlook 2000 SP3, Outlook 2002, and later. Also manages optional settings such as minimize to tray and purge data file on exit. New settings added in new Outlook updates are supported through online updates to the software. (Formerly Xenos Outlook Security Extension)
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.