This is not dogma - you should write to reflect your own personality - but will hopefully make you think about things you didn't think about before, and help you maximize your email effectiveness.
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 email documents, your recipient can ask questions immediately. Email thus tends, like conversational speech, to be much sloppier and more ambiguous. (See also a quick essay on formality that I am trying to figure out how to work in to this document...)
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.
Granted, you should put some effort into keeping your subjects agreeing with your verbs, spelling correctly, avoiding mixing metaphors, and so on. But if The Rules that Mrs. Grundy laid down in seventh-grade English get in the way of effective communication, throw them out.
However, because of the lack of vocal inflection, gestures, and shared environment, email is not as rich a communication method as a face-to-face or telephone conversation. Your correspondent may have difficulty telling if you are serious or kidding, happy or sad, frustrated or euphoric. (Sarcasm is particularly dangerous to use in email.)
Thus your email compositions should be different from both your speech and your paper compositions. There are a fair number of documents on electronic email out there, but when I finished the original version of this document, they mostly talked about the nuts and bolts of how to get text from your fingers to your correspondent's screen. Those that did discuss email content tended to be really brief on the subject of email style, and gave little motivation for why the style is different. That's why I wrote this document: to address email style.
Since I wrote this, I have found two documents which cover a lot of the same ground. If you want more and/or a different perspective, see: