I am experimenting with forms for the first time. I would like to create a
new form with a custom drop-down field.

I created a new mail and post items folder for my excercise. I then designed
a new form based on the Message form, saved and published it (in the new
folder). I then went to the folder properties to set the new form as the
default on the general tab. Unfortunately, when I try to select my custom
form from the "When posting to this folder, use:" drop-down menu I receive a
"You cannot create an item of this type in this folder" error message.

What am I doing wrong??