I programmatically add a user property to an appointment before I send it to
a recipient. This is done via a Windows Form (using VSTO) launched from an
Outlook tool bar.

appointment.UserProperties.Add("ExamplePropertyNam e",
Outlook.OlUserPropertyType.olText, true, Outlook.OlUserPropertyType.olText);
appointment.UserProperties["ExamplePropertyName"].Value =
"ExamplePropertyValue";
appointment.Save();

However, the user property is not propagated to the recipient. I am
wondering if it is related to this KB article:

http://support.microsoft.com/kb/907985

But adding custom properties programmatically is listed as a best practice
and I am not using Outlook forms at all.

Do I have to use the AllowNamedProps registry key as well?