In a conversation, there is some minimum of shared context. You might be in the same physical location, and even on the phone you have, at minimum, commonality of time. When you generate a document for paper, usually there is some context embedded in the medium: the text is in the proceedings of a conference, written on a birthday card, handed to your professor with a batch of Econ 101 term papers, or something similar.
With email, you can't assume anything about a sender's location, time, frame of mind, profession, interests, or future value to you. This means, among other things, that you need to be very, very careful about giving your receivers some context. This section will give specific strategies for doing so.