For a custom message form with custom properties to work fully, the form must be published to the Organizational Forms library or to each user's Personal Forms library, with the "Send form definition with item" box on the (Properties) page of the form design unchecked.
Simpler forms without custom properties will work correctly.
For more information, see Changes to custom properties in Outlook (//support2.microsoft.com/kb/907985)
Published November 12, 2008. Last reviewed on June 17, 2011.