2. Never send and preferably don't even try to write an email if you're angry, upset, drunk, or otherwise not in total control. If you have a heated conversation with someone on the telephone you can sometimes fudge things over. But with emails, once you hit "send" whatever you've written is there, carved in tablets of stone, for as long as the recipient wants to glare at it. The old adage about "counting to ten" before responding couldn't be more true here. Only send angry emails if you can handle, or really don't care about, the recipient's resultant feelings!
3. One thing that you may not think of is that it can be useful to consider carefully the time you send your emails. To begin with it's always a good idea to avoid sending emails that coincide with the Monday morning rush and Friday afternoon lethargy. In addition, I've occasionally found that emails sent to companies over the weekend end up getting lost in cyberspace. And on a rather more subtle level, if your recipients see that you're sending emails on a Sunday morning or late at night, they may feel they can interrupt you for a business talk at the same times. Although you may think it's cool to impress a client that you work all hours, your partner won't when the same client calls you on the phone at midnight.
4. Because almost everyone at some time or another has been infected with a computer virus, people are understandably wary of attachments. I never send attachments to anyone I don't know very well, and equally never open attachments unless they're from people I know well. And then, some contemporary viruses and worms clone themselves on to genuine email names and addresses, so even an email purporting to be from someone you know might just be infected. When in doubt append text to the body of your email message, or contact the recipient beforehand and make sure they're happy to receive it as an attachment.
5. Layout of emails is something few people pay attention to, especially if (like me) their system uses text only. However even with simple text a sensible layout can make the whole thing more readable. Above all, you should avoid writing emails that sprawl all the way across the screen. Those are very hard to read and to be able to see everything properly as text, your reader may have to fiddle about changing fonts. The safest format to use consists of lines no more than 65 characters long. That fits, works everywhere and makes the email much easier on the eye.
6. Your subject line should focus on what's in it for the reader so it grabs their attention. You'll find that the best way to do that is to include some sort of benefit. For example, if you're writing an email about a downwardly-revised project budget, instead of saying "Project X -- revised costs" say "Project X -- costs reduced by XX%"). If there isn't a genuine benefit to use, try to make it interesting and intriguing anyway. Also, avoid the words most hated by spam filters like "free," "subscribe," etc.
7. Online writing has to be kept concise and clear, largely because the screen is a particularly unfriendly reading medium for most people's eyes. If only for that reason the KISS principle (Keep It Short & Simple) is useful. With emails you need to get straight to the point and keep to it. Someone who receives dozens of emails per day doesn't have time to wade through a lot of preamble. By making your point concisely you'll stand the greatest possible chance of avoiding the undignified fate of being deleted.
8. As far as writing style is concerned, here more than with any other medium it's very, very helpful to write as people speak. In addition, it will make your email clearer and more concise if you leave out all but essential adjectives and adverbs. Keep your sentences short, and only ever include one main idea or thought per sentence. Paragraphs shouldn't consist of more than 6 sentences max -- fewer if possible. And if you list more than a couple of items, use bullet points.
9. If you write emails for business, make good use of the signature facility that goes after your name. It's surprising just how many people fail to use that facility properly - yet it's an excellent opportunity for you to put across a few words of promotion. Because the email signature appears at the end, your recipients are not likely to be irritated by it. In fact provided that it contains useful contact information it will be seen as a helpful addition to your message. And even if your email is text only you can still make it look reasonably smart.