It controls whether VBA procedures can run without prompts and, depending on the value of the DontTrustInstalledFiles value, what COM add-ins can load. <plug>These are covered in detail on pages 249-252 in my latest book (see signature)</plug>.
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Sue Mosher, Outlook MVP
Author of Configuring Microsoft Outlook 2003
and Microsoft Outlook Programming - Jumpstart for
Administrators, Power Users, and Developers