Assuming you already have the action you want to use installed, you'd choose as one of your rule actions "perform a custom action".
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Sue Mosher, Outlook MVP
Outlook and Exchange solutions at http://www.slipstick.com
Author of
Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers
Custom actions are separate components, not part of Outlook. If you want to use one, you have to download and install it. See http://www.slipstick.com/addins/custom.htm for a list of those known to be available.
--
Sue Mosher, Outlook MVP
Outlook and Exchange solutions at http://www.slipstick.com
Author of
Microsoft Outlook Programming: Jumpstart
for Administrators, Power Users, and Developers