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If you have one of the processed messages that didn't create an action, select it in the Mail app, then do View->Message->Raw Source and look for the Content-type: line. If it doesn't say "text/plain" it isn't going to work.

Good: Content-type: text/plain; charset=US-ASCII; format=flowed

Bad: Content-type: multipart/alternative; boundary=Apple-Mail-3-62950437

Also make sure your email address you are sending from is on the list of acceptable addresses in the Mail preferences! That and the rich text problem are the two most frequent causes of this not working, I think. You'll see exactly the behavior you are seeing with either (or both) of these issues biting you.