1.) Every email should contain your full name in it somewhere and, if you have a question about class, you should refer to the class by name.
2.) Use subject headings to help your recipient. The subject "Hey," for example, invites deletion. Write something that addresses the content of the email. Do not continue the subject heading into the email. Address the subject AGAIN in the body of the email.
3.) In a long email exchange, change the subject heading to acknowledge the movement of the "dialogue." A series of "Re:Re:Re:" is not helpful at all.
4.) Using "FYI" to pass on an email is simply a lazy way of dumping information into someone else's email inbox. If you think the information is valuable enough to pass on, do the courtesy of providing your recipient with a context that make sense of why this particular bit of "information" is valuable to them! And don't just copy and paste an entire message. Clean the email up and get rid of unnecessary (and distracting) metatextual coding, lists of recipients, and other extraneous material. Make YOUR note--your FYI--clear and cogent.
5.) Try to check your spelling and syntax in emails. Email is a fast medium, so errors will occur, but email itself shouldn't be an excuse for letting errors get through.
6.) Let your recipient know if you're copying someone else on the email. If you are writing to a lot of people, you should use BCC, so that the names don't all appear in the header... but again, let your recipient know that a certain group is being copied.
7.) Create a signature for yourself, which includes information your recipient might need to know. If you include a "pithy" quote, try not to choose something too precious or too sententious.
8.) Rather than use "emoticons" to alert your reader to humor, irony, or concern, try to articulate your views in the best possible and most suggestive style.
9.) Think before you write and reflect before you hit "send." Email may be a casual form of communication, but it is very powerful. It can hurt people, misrepresent you, and be misconstrued. It can also find its way to many recipients beyond your intended recipient and get back to you in ways you never intended.