Here are some rules about email. They represent only my opinion, but I think they're pretty good rules.
Don't top-post. The best way to describe what top-posting means is to look at a standard reply generated by Microsoft Outlook Express. On hitting ‘Reply’, Outlook Express quotes the original message, but places the user's signature above the quote, and the cursor above that. (Additionally, in HTML-mode, it quotes the original message by placing a vertical bar to its left—worse, it seems impossible to break that bar to insert meaningful responses to the original.) Why is this bad? It's bad because it increases the difficulty of replying to the original in a meaningful way. If the original contained more than one question (or more than one statement requiring a response), then to make the most sense, the answers should be interspersed throughout the original. An answer should be below the corresponding question. Why is this good? Because the respondent won't miss any of the questions, and a third party can pick up the train of the correspondence in a single read-through.
Answer all the questions. If someone has asked five questions, they want five answers. There is nothing more infuriating than composing an elaborate email with numerous questions and getting ‘Yes.’ as a response. (Even worse is a top-posted ‘Yes.’ See above.)
Make your mail legible. Some of the things that make mail illegible include over-long lines, and a lack of vertical spacing. (That is, at the very least, put a blank line between paragraphs.) Greg Lehey has compiled a list of the most offensive layout problems.
Write in full, grammatically correct sentences, and check your spelling. This is particularly important if you don't have a pre-existing relationship with the recipient, or if you are posting to a widely distributed mailing list.
Don't send HTML mails. Email pre-dates the World Wide Web by quite a few years, and existed perfectly well without HTML. Mail should be sent in plain text.
Set your system clock and the correct timezone. Most people display their inbox in some kind of chronological order. If your clock is wrong, or sometimes even the timezone, your email can easily get ‘lost’ since it won't appear at the ‘front’ of the list when it arrives.
Don't send a mail on a new subject as a reply to a previous message, send it as a new message. Some mailers allow the reader to show a list of messages as ‘threads’. (That is, roughly, groups of messages sent as replies to each other.) Even if you change the subject, your message will almost certainly appear in the original thread because it will have an In-Reply-To header. At best, this will irritate the recipient. At worst, the recipient won't see your mail.
Reply. Email is a fairly instantaneous medium—an email does not take long to compose, and it takes even less time to send and arrive in the recipient's inbox. This observation carries with it, perhaps slightly unfairly, an expectation that replies will be similarly quick. Of course, what constitutes a timely reply might vary from case to case. If you're conversing with a friend, a reply might reasonably wait a day or two. If you are using email to conduct business transactions, replies are probably better sent as soon as possible after receipt. Regardless, if you can't reply in full straight away, at least send a short note to acknowledge receipt.
Always check the ‘Cc:’ list in any mail you receive, and both the ‘Cc:’ and ‘Bcc:’ lists in any mail you send. Always.
With incoming mail, check whether you're the main recipient (that is, whether your name appears in the ‘To:’ list), or whether you're receiving a copy. If your name appears in the ‘Cc:’ (carbon copy) list, you're obviously receiving a copy—the implication here is that the email is primarily directed to someone other than you, but that the sender thought it would be relevant to you in some way. If you name appears nowhere in the headers, you've almost certainly been sent a blind carbon copy of the mail (that is, the sender put you on the ‘Bcc:’ list). Reasons for doing this vary, but it's usually because the sender did not want the listed recipients to know that you were also receiving a copy of the mail—otherwise the sender could have just put your name in the regular ‘Cc:’ list. At this point, stop and think before you hit ‘Reply to all’. While you can reasonably reply directly to the sender, it will almost always be highly undesirable to reply to the listed recipients.
If an incoming email has multiple recipients (and you've established, as described above, that you were not on the ‘Bcc:’—that is, everyone else knows you're a recipient of the mail), then the default action on replying should be to ‘Reply to all’. After all, the sender obviously had a reason for naming multiple recipients. If it was appropriate for all those recipients to read the original, it's probably appropriate not to fragment the communication—copy them on the reply.
Having described the default action on replying, always be ready to override this default at any time. In other words, always double-check the ‘To:’, ‘Cc:’ and ‘Bcc’ lists in any email you send, whether it's a reply or a new composition. Is the content appropriate for each and every listed recipient? If in doubt, trim.