Keep it Brief and to the Point
“Brevity is the Soul of Wit”
– so said Shakespeare. Who are we to argue with Shakespeare?
Don’t Check Email Compulsively
Check email once or twice a day. Check email when you have enough time to properly respond. If you don’t have time you may rush a reply and not give yourself time to think. Don’t have email on auto prompt. – I.E. every time a new email comes you get a prompt, – you will never get anything done.
Not matter who you are replying to, always try to be polite and cordial. Remember any negativity can get magnified in an email.
Don’t Say anything you wouldn’t say in Person.
The problem with email is that the personal contact is lost. Therefore, we feel detached, sometimes this can make it more willing to say things we may regret. When writing an email, feel you are actually speaking to the person.
DO NOT WRITE IN ALL CAPS
IT LOOKS LIKE YOU’RE SHOUTING. PLEASE TURN IT OFF!
Explain your Problem Carefully.
I could write a whole post on how to report technical faults. The important thing is to give as much detail about what has gone wrong. Bearing in mind the sys admin is not psychic. Avoid “It doesn’t work! This is useless!” Look at How To Write a Bug Report
Don’t Respond When Angry
American President, T. Jefferson said
“When angry count to ten before you speak. If very angry, count to one hundred”
May I add “When angry and emailing count to one thousand.”
Let Emails Wait
If you have a difficult email to write, before sending, let it wait overnight. Go back the next day and revise the contents. Often you may wish to moderate your comments.
Do you Really Need to Write the Email?
Some emails are essential, but some are non essential. If we have the opportunity to speak to somebody anyway, there is no need to send a stream of emails about general chit chat.
Good English Should not to Be Forgotten.
Maybe I am bit old fashioned, but when writing emails I like to maintain the basics of good spelling and punctuation. (Mozilla inline spell checker is a real Godsend)
Some people feel they don’t have time to use Capitals, spelling and the basics of grammar. However, it is likely that they are writing more emails than necessary. If you don’t have time to write emails properly it is better to try and write less.
Break up your Email into bullet points
If your email is long, or contains several points, don’t write in long one sentence. White space gives the reader a breathing space. If you break up the mail into parts it makes it more likely it will be read. Note, how this post is broken up into different parts.
If you have many people writing about the same thing, write a standard reply you can customise. Save a short note in word or in an email draft. You can then add this standard reply to all relevant emails, saving time.
Give Email a relevant title
Makes for easier scanning. Don’t fill title with Re:re:re. If the subject changes, change the title.
Keep it short.
Did I mention keep it brief? The email should have a clear purpose. Avoid going off at a tangent. If you want to ask about “how to Create a new folder”. There is no need to start relaying a list of complaints about your new PC (or Mac) 
When email seemd very new (back in 1997) I was a student at Oxford University. We used to sometimes joke about political views, which were not entirely politically correct. After joking around, I decided to write an email and then, without thinking or reviewing it, I posted the email to the Guardian Newspaper.
They only went and printed it ! I was horrified to see this email, published in 8 million newspapers (slight exaggeration) but that is what it felt like. If it hadn’t been so easy to press the send button I may have held back and not sent it.
(I cannot at all remember the subject of the email. But, for example, it could have been a topic like men being more intelligent than women) – The point is, I learnt, you have to be careful about writing emails.
 Of course Mac users would never complain about their new computer – Even though it doesn’t have a delete key, which would be very useful.
Books on Writing Emails