Designing custom forms in Outlook 2000

I can't get my custom form to be sent and displayed.
Checking the box on the properties page: "Send form definition with item" should show the custom form to the recipient of the email.
However, it doesn't work for me!
My message is based on the Message form, so it substitutes the standard mesage form to the recipient (message class IPM.Note) because the form definition is NOT being sent with the form.
Have I missed something?