By default, access is restricted to certain e-mail message attachments in Microsoft® Outlook®. In determining which attachments are blocked, Outlook checks the file type of the attachment. Files with certain file types can be completely blocked (Level 1) or the user may be required to save a file to disk before opening the file (Level 2).

By default, Outlook classifies a number of file type extensions as Level 1 and blocks files with those extensions from being received by the users. There are no Level 2 file types by default, but you can create a list of Level 2 file types by using the Outlook Security template. You can demote file types from being categorized as Level 1 to being Level 2, or you can add other file types to create a Level 2 list.

 Note   To learn more about why some attachments are blocked, see Blocked attachments: The Outlook feature you love to hate. In addition, users can find methods for sharing files that are blocked by Outlook by reading About unblocking attachments.

Level 1 file types blocked by Outlook

The following table lists the Level 1 file types that are blocked under a default installation of Microsoft Office Outlook 2003. You can add or remove items from the default list through the Outlook Security Settings tab of the Outlook Security template.

The following table lists the Level 1 file types that are blocked under a default installation of Microsoft Office Outlook 2003 Service Pack 3 (SP3).

Level 2 file types restricted by Outlook

There are no Level 2 file types by default, but you can create a list of Level 2 file types for your deployment. Level 2 file types are restricted (under a default installation of Outlook 2003) and you must first save a Level 2 file to disk before you can open it —Level 2 file types cannot be opened directly from an item in an e-mail message.

As an administrator, you can add or remove attachment types from the list of Level 2 file types through the Outlook Security Settings tab of the Outlook Security form. Also, you can demote attachment file types from Level 1 to Level 2 by using the Level1Remove registry key. For more information about setting the Level1Remove registry key, see Administrator-Controlled Settings vs. User-Controlled Settings.