These really only apply to e-mail destined within your company or folks that you work with. Customer or vendor correspondence should be handled differently.
Identify your intended tone
I (unconsciously) use an eleven point scale point scale from -5 to +5. Zero is neutral. If you intend your tone to be negative, consider whether or not it's a good idea to sleep before you write. If you wake up and still feel the need to pull the trigger, perhaps you should pick up the phone lest you end up here. Lose the passive-aggressive communication and have a real one-on-one chat. If it's done right: It won't be comfortable and it will affect both of your perspectives.
Always address the individuals you're writing to
If you're sending a message to one person, start it off with a simple first name and a comma. "Bob," not "Dear Bob" (is he really dear?).
If you're sending to a few people and CCing a few people, use the names of your "To"s (John/Tom,). If you're sending to a large group, don't skip the opening. "Greetings, Good Morning," even "Howdy," (with the right group of people), even "Hi!" is acceptable in the right context.
It comes down to tone, again.
Of course, use discretion. If you're dealing with a long thread that you have already contributed to before, it may actually be better to skip the greeting, lest you sound like you're targeting.
Use Good Subjects
At a minimum, include a subject. Secondly, put some thought into the subject. Thirdly, at least capitalize the first letter of the first word of your subject.
After you're done writing your e-mail re-read it. Then compare your subject to the content of your message. Rewrite it if you can't connect the two.
Check the subject on replies
Is the subject of the message you're replying to the 30th message? Is the original subject still remotely relevant to what you're replying to? Just because you're replying doesn't mean that you shouldn't have a useful, relevant subject. Modern e-mail clients have allowed us to neglect the subject on reply.
Stop the pattern of destruction. Change the subject when the Re:... doesn't make sense. As a bonus, use the well forgotten (was Re:...)
Idiotic scenario: your colleagues have been discussing how best to handle ordering buffalo style chicken wings for the annual "summer is officially over" lunch and 30 replies later everyone is settled on getting a variety of wings at spice levels from "vinegar" to "$#%@", but as the guy who had to replace the keyboards after the great buffalo wing debacle of 2003, you have something to add...
Original, and well worn subject:
Re: buffalo wings
Your reply 30 messages later:
Wet naps (was Re: buffalo wings)
Being an individual who handles a ridiculous amount of e-mail, sometimes one or two of those thirty messages get missed. A subject change with reference to the original topic helps to identify the branches of the tree and might result in your message being read.
Formality and signatures
I have no source to cite because this is my blog and it simply isn't important enough. Sorry!
The end of your message is very important. Much like the opening, it can change the tone of the entire note you've sent.
Again, focusing on internal communications only, remember that "you're all supposed to be on the same team". If you're asking for project statuses, or a request of any kind, it can come off as pushy. Assuming you're trying to get someone from outside of your particular sphere of influence to do something that will only benefit the project you're working on, consider toning down the formality.
My personal rule is that I don't have an automatically appended "sign-off" on my outgoing mail. After the "double dash", the remaining words are only glanced at by your recipient ... much in the way your recipient might glance at the floor or something less interesting. If the impression they receive is of a guy in a suit with his arms crossed in front of him, the tone of your message will be one "from authority" ... if so, I hope you have enough of that authority to prop up your written communication. If the picture is of a guy offering a hand to shake with a smile accompanying it, the tone of your message will be friendly.
People will exceed their normal boundaries to handle a request if it's delivered on friendly terms and common ground. "Take care! Thanks! Cheers!", rather than "Good day. Sincerely, Thank you," is the way to go.
The purpose of this isn't to encourage one to manipulate others. It's to help bridge the gap between verbal and written communication. If you're not being intentional about tone in your writing, you are probably being misunderstood.
I'm no verbal or psychological master (read my blog if you don't believe me). So take my advice with a grain of salt all ye three that have read this page in the last month :o).