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You need to write and tailor your writing style to the person/organization who's receiving the message and the purpose of the message. If you're writing to a friend and acquaintance, you can use an informal style and language that can include slang words to get across your message. If you're writing to a business, you want to express yourself more formally and choose your words carefully to ensure clear communication.
If you share the purpose of the email, we might be able to better help you.
I find that the best thing about emails is not how long and well written they are, but how well you communicate the topic/subject of the email.
Sometimes, less is more.
Emails should be short.
Even if they are long, you should be thinking that. So always get directly to what you are saying or asking.
If they ARE long, then you want them to be readable. Try to limit your typing to three sentences per paragraph. To do this, try to make each paragraph about one topic - then move onto the next.
Also, they are e-*MAIL*. So, like regular mail, you should include the receipt(s) name(s) first, then write, then sign yourself as the sender.
Hope this helps,
P.S. They way this post is written is an example for you!