Electronic communication, because of its speed and broadcasting ability, is fundamentally different from paper-based communication. Because the turnaround time can be so fast, email is more conversational than traditional paper-based media.
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 sloppier than communications on paper.

This is not always bad. It makes little sense to slave over a message for hours, making sure that your spelling is faultless, your words eloquent, and your grammar beyond reproach, if the point of the message is to tell your co-worker that you are ready to go to lunch.

However, your correspondent also won't have normal status cues such as dress, diction, or dialect, so may make assumptions based on your name, address, and - above all - facility with language. You need to be aware of when you can be sloppy and when you have to be meticulous.

Email also does not convey emotions nearly as well as face-to-face or even telephone conversations. It lacks vocal inflection, gestures, and a shared environment. 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.

Another difference between email and older media is that what the sender sees when composing a message might not look like what the reader sees. Your vocal cords make sound waves that are perceived basically the same by both your ears as your audience's. The paper that you write your love note on is the same paper that the object of your affection sees. But with email, the software and hardware that you use for composing, sending, storing, downloading, and reading may be completely different from what your correspondent uses. Your message's visual qualities may be quite different by the time it gets to someone else's screen.

Thus your email compositions should be different from both your paper compositions and your speech.