If your environment uses public folders, and if you use public folder security forms in earlier versions of Outlook, you can continue to use public folder security forms. You can do this after you make a minor change to the appropriate registry settings. Also, Outlook 2007 is designed to take advantage of the GPO settings to manage security for attachments and for add-ins.
More detailed information can be found in the following article.
Information for administrators about e-mail security settings in Outlook 2007
For your convenience, we have listed the previous article of “public folder security forms” of Outlook 2003 below.
How to use Outlook E-mail Security Administrative Package to Trust Add-in or Other Programs