I get the 'not delivered' error whenever I send an iMessage from my iPad (which uses my email address). But when I send an iMessage from my iPhone (where it uses my number, obviously) it works perfectly. No errors. Also, whenever I iMessage from the iPad, the recipient also gets the 'not delivered' message when trying to reply to me. But the weird part is, there is really no error because we can see each others messages perfectly. They're coming through but saying 'not delivered'.