When it comes to writing your e-course, you should think about the messages as being chronological steps that someone should take to reach an end goal. For example, let's say that your e-course is called Mastering Twitter in 30 Days. Your e-course might then deliver thirty e-mails over the course of thirty days and each day contains a different to-do or a different tip (or both).
Your e-course purchasers / subscribers should be able to go along on a journey with you and feel like there are actionable items available to challenge them on the subject they signed up to.