Electronic communication, because of its speed and broadcasting ability, is fundamentally different from paper-based communication. Because the turnaround time can be so fast, e-mail is more conversational than traditional paper communications. In a paper document, it is absolutely essential to make everything completely clear and unambiguous because your audience may not have a chance to ask for clarification. With e-mail documents, your recipient can ask questions immediately.
Email thus tends, like conversational speech, to be much sloppier and more ambiguous.
This is not always bad. It might not be a worthwhile expenditure of energy to slave over a message, making sure that your spelling is faultless, your words eloquent, your grammar beyond reproach, if the point of the message is to inform the recipient that you are ready to go to lunch. But you can at least put some effort into keeping your subjects agreeing with your verbs, spelling correctly, avoiding mixing metaphors, and so on. Your correspondent may have difficulty telling if you are serious or kidding, happy or sad, frustrated or euphoric. Thus your e-mail compositions should be different from both your speech and your paper compositions.