With Outlook, each major release and service pack includes new Windows registry settings that add new features or give you greater control over the way the client software operates. Because you can easily lose track of which registry entries are available, let's review some of the most useful entries for Outlook 2002. If a particular subkey or value I mention doesn't already exist in the registry, you must create the entry to enable the feature I discuss. Before you implement any of these registry changes, I recommend that you read any relevant Microsoft articles, back up the registry, and test thoroughly.
How can I control which attachments Outlook's security feature blocks?
Most administrators know that Outlook 2002 supports a Level1Remove string value in the HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security registry subkey. If you enter a semicolon-delimited list of file extensions (e.g., url;lnk) for the Level1Remove value, users can access and save file attachments that Outlook usually blocks. However, note that if the value DisallowAttachmentCustomization exists in the HKEY_CURRENT_USER\Software\Policies\Microsoft\Office\10.0\Outlook subkey, attempts to change to the Level1Remove value have no effect.
Outlook also supports a Level1Add string value, which lets you enter a semicolon-delimited list of file extensions to add file types to the blocked list. This feature doesn't work in the release to manufacturing (RTM) version of Outlook 2002, but Microsoft Office XP Service Pack 1 (SP1) restores the functionality. For more information about these changes, see the Microsoft articles "OL2002: The 'Level1Add' Registry Key Is Missing from Outlook 2002" (http://support.microsoft.com/?kbid=312834) and "OL2002: You Cannot Open Attachments" (http://support.microsoft.com/?kbid=290497).
Of course, you can also use administrative tools to control attachment blocking. The Outlook 2002 security form includes an option that lets you stop users from customizing attachment security settings. You can also use a policy to prevent users from customizing attachment blocking. For more information about these tools, see Outlook Tips & Techniques, September 2000, http://www.exchangeadmin.com, InstantDoc ID 9581.