Write around 10-20 lines of text, each comfortably 72 characters long, at most 76 characters.
Do not allow your environment to add any of the following to your message:
If your environment does add any of these to the message, turn that feature off. If you can't turn it off, use a different environment. If you cannot obtain e-mail sending services for free (often, your Internet Service Provider will also offer you an e-mail account), pay for them.
You do not have to prove that you are hip.
Do not show off by artfully misspelling words; nor must you indicate your extreme relaxedness by leaving off punctuation, capitalization, or vowels.
Strive to follow the rules of the language you are using. For example, when sending English, do not use "u" to abbreviate "you", or "plz" to abbreviate "please". Use upper- and lowercase appropriately. Use punctuation and paragraph breaks to make your text easy to read.
If you don't know the gender of your recipient, address them formally as "Dear Sir or Madam," or informally with their first name, or just "Hi," or by referring to the role you're addressing them in. ("Dear Maintainer of www.example.com" or "Dear Mycompany Customer" is a perfectly acceptable greeting.)
If you approach someone you do not know personally, your first sentence should describe how you know their address. If you learned their address from a webpage, quote the URL of the webpage.
Do not use online contact management services to invite people you don't directly know into correspondence with you. Ask them first, in personal e-mail; then invite them if the system requires it. Similarly, do not send online postcards to people you don't know; do not use "Mail this to a friend" links, either.
When sending URLs in e-mail that is intended to mostly convey those URLs to the recipients (i.e., you saw something exciting that you want to share), send along a natural language description of what the links refer to, even if it robs your recipients of the surprise that you yourself just experienced.
If you're concerned about the URL's length, send it broken up with white space and newline you provide, and accompany it with a shorter alias (as provided by, e.g., tinyurl.com and makeashorterlink.com) that your recipients can click on without fixing layout damage first.
Don't send items or references to items whose source you do not know. Never, ever, send anything to "everybody in your address book".
When replying to a message, quote it and either append it to a very short comment; or, if you have longer comments, intersperse your comments with quoted excerpts from the original.
Do not append a very short reply to a very long message; if what you say is short, move it to the front.