This is the first in a series of four. It's been sitting in my archives since 2007, and I've been putting it off further and further, refining it here and there. I was going through my blog drafts, and thought I should finalize this, and post it. The other two will come in short order.
I'm subscribed to a great deal of mailing lists. Twenty-nine, actually. I receive upwards of nearly 200 personal emails per day. Not only personal email, but work mail too. In fact, with work, it approaches 500 per day. Above that, I spend A LOT of my time writing email. I'm not saying this to come off as some sort of expert on the subject. Rather, I know what bothers me with email, as well as what bothers others. I've seen this discussion over and over on mailing lists and even personal email. So, here's what I would say would be a sufficient list of "email netiquette".
My target audience is specifically those who are on technical mailing lists. However, I feel these netiquette rules can also apply to personal and corporate email as well. Here's a brief summary. I'll expound on each individual item later in the post.
- Use plain text (preferred) or HTML
- Top-post only when forwarding. Bottom-post otherwise.
- Trim your replies.
- Keep you signature under five lines, and use the signature separator "-- " (dash, dash, space).
- Do not attach unnecessary files, keep attachments small, and don't attach proprietary formats.
- Keep the width of your message under 80 characters
- Use a client that sends threading headers.
- Reply only to the necessary people (don't abuse CC: or "reply to all").
- Be short and concise. Don't ramble (stay on topic).
- Break up your paragraphs.
- Use proper spelling, grammar and punctuation (avoid CAPS).
- Don't answer spam, and don't send out spam.
Use plain text (preferred) or HTML.
Most email conversations I've ever seen have never needed to be composed in HTML format. There was no need to embed an image to clarify the topic, format text to emphasize meaning, or use a fancy layout to enhance the message. Granted, corporations such as Amazon, Bose, Newegg, and others will send HTML email, completely formatted like a web page when a customer makes a new purchase, and they wish to send a receipt or tracking information. Personally, I think all that extra bloat is just that- bloat. The critical information is the text, not the images or layout. In fact, whenever I see an HTML email, 99.99% of the time, it could have been composed in plain text, and same meaning would have been conveyed.
The first problem with HTML email, is not everyone is using an email client that is capable of parsing HTML cleanly, or even at all. Plain text is the universal format. Every email client can parse plain text. Second, most cases of HTML email I've seen add a great deal of bloat to the size of the message. Especially when people add background images to "personalize" their messages. It's unnecessary, and when I have seen these "personalized" messages, they're not done cleanly. They're throwbacks to the 1990's when no one knew how to properly design a web page, so they just look hideous. Do you and everyone else a favor, and ditch the fancy fonts, background images, and animated signature.
However, with that said, I understand that there is a time and place for HTML email. There are times when you need to compose a table, which plain text will not display accurately. There are times when using a bulleted list, italic or underlined words are necessary. There's a time to sparingly use color. Sometimes, embedding images into the mail can add to the message the author is trying to convey. I don't have any problem with this. After all, composing an email document is just that- a document. It's word processing. There is a time and place for HTML email, and I accept that. Its need is usually rare, however, so plain text will probably suffice in most cases.
On a side note, one piece of netiquette that really doesn't deserve it's own subject, but can fit well here, is to avoid embedding URLs in the text. Instead, make reference to the URL with footnotes. For example:
Please refer to the documentation  for help on configuring your printer. Release notes for the software might be helpful with configuration as well 
You can put all of your footnotes at the bottom of your message, or you can interleave them, as Debian usually does with their weekly news.
Top-post only when forwarding. Bottom-post otherwise.
When reading English, you read left-to-right, top-to-bottom. Email threads are the same way. When you join a discussion, you expect to read the question first, followed by the answer. Blog comments, web forums, BBS, IRC, Facebook, Identi.ca threads, on and on. People read top-to-bottom naturally to follow a conversation. Not bottom-to-top. However, for some strange reason, reading the answer first, followed by the question has become acceptable in email. I'm not sure how this happened or why we continue to put up with it, but this is perhaps one of the most disturbing aspects of email threads. Case in point:
A: Trim the message, leaving appropriate context, then reply below.
Q: How should I reply to email then?
Q: Should I include quotations after my reply?
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
Q: What is the most annoying thing in email?
I blame Outlook users and people who use web-based email such as Gmail, Yahoo!, Hotmail, etc. for this gross perversion of the Internet. Some reasons top-posting sucks:
- Top-posting is lazy. Just because your client puts some extra carriage returns before the original text, and starts your cursor at the top, does not mean it's acceptable. Press your delete key a few times, then press CTRL+END to get your cursor to the END of the message, and start typing there. We're talking 3-4 keystrokes for appropriately replying to email.
- Top-posting is arrogant. Top-posting means that I want to read your reply first, before I get the full context of the message. It means the reply should be heard, before the argument. In spoken conversation, we would call this interrupting. Your message (spoken or written) is far more important than what you're replying to. It's just plain rude.
- Top-posting also encourages email bloat, with message, after message after message beneath the reply. I'll cover trimming your replies in the next section.
- Top-posting generally means that the person relpying has not read the thread, or possibly even the message they're replying to. In my experience in the world of email,
However, there are times when top-posting is appropriate, or at least acceptable. When forwarding emails to another individual, it's generally a good idea to tell the reader know what you've forwarded before they start reading the message. If they read the message first, and you had put your message text beneath the forwarded email, telling them what the message is about, it would be provide no benefit. So, top-posting when forwarding is certainly appropriate.
Top-posting is also acceptable in corporate environments, where the culture is to top-post (usually, because of Outlook). If you were to bottom-post all of your replies, you might throw off your readers who are expecting to see your reply at the top of the message, and not the bottom. In such an instance, you would be the scourge of everyone else, making for a possible uncomfortable working environment.
Lastly, top-posting a reply can be acceptable when you want to add information to the thread before you begin adding your reply. Things such as a warning of your tone, adding a new person to the thread via CC:, then summarizing the thread before the reply, or giving information about your reply before hand would be appropriate. So, while top-posting is generally discouraged in replies, there are a few times when it's okay. However, try to keep bottom-posting your replies the default behavior.
There's even an RFC that addresses appropriate posting style. RFC 1855.
Trim your replies.
Trimming your replies means to leave only the appropriate context that you are replying to, cutting extra cruft out of the message. This is another problem with top-posting, that I briefly mentioned above. When people top-post, they leave every reply in the thread in the message. The more people top-post to the message, the longer, and longer the message gets, adding a great deal of bloat. When people don't trim their replies, they remind me of pack rats- people who continually store stuff in their house, even though they'll never, ever see it or use it again, they have to have it "just in case". Their house gets cluttered wall-to-wall, floor-to-ceiling with crap. Not trimming your replies is similar in nature.
Let me give you some examples, so you can clearly see what I'm talking about. I'm subscribed to the Chromium Discuss mailing list. Due to the topic of the mailing list, many people are big Google users. As a result, many are using Gmail as their primary account. Not just using the account, but using the Gmail web interface. By default, the Gmail web interface will put a couple of new lines before the thread you're replying to, and place your cursor at the top of the message. So, as you can imagine, just about everybody top-posts. It's not 100%, however, as few people, such as myself, use an external client such as Thunderbird or Evolution, which encourage bottom-posting by default. So, the mailing list sees a mixture of bottom-posting and top-posting, with more top-posting than bottom.
Here's an example of a thread about someone having trouble setting their router password with the Chromium browser. Everyone in the thread top-posted, and did not trim their reply. So, the email message grew and grew in size. Everybody's inbox is being wasted, due to the inconsideration of all those top-posting, because of the massively redundant data.
Further, imagine someone just joining the list for the first time, and this message is the first one their receive as a new member to the list. They have to read the message in a convoluted upside-down top-to-bottom, bottom-to-top manner to get what is going on with the thread. Top-posting, as mentioned previously, is not appropriate, and should not be tolerated on mailing lists.
Now, let's look at an example of someone who trimmed their reply. Here is a reply about not updating the release calendar for Chromium. The subject tells the reader what is initially being discussed. The author trimmed the reply, leaving appropriate context for his reply. The context was that the calendar will be updated "tomorrow", and his reply with "Thank you". All the necessary information is there, and if a reader were to just join the list, and this was their first email, they would know enough about the thread, without having every message present.
As with top-posting, not trimming your replies is lazy, and again, it's rude. Some people don't have the hard drive space you might, or the bandwidth to pull down such a noisy message. Cutting out the cruft, leaving the relevant pieces in, is considerate, polite and logically sound. Do you, and everyone else a favor, and trim your replies.