(1) Read all unread messages in your inbox, to avoid sending
  superfluous messages.

(2) Think before you write. Is your message relevant and appropriate?

(3) Think after you write. Re-read your message. Is it clear, concise
 and (again) relevant? Off-topic comments (sometimes flagged OT in
 the subject line) may be acceptable in some online communities,
 but not in others.

(4) Write properly. Many people will not take you seriously if you  write
 messages without capitalization or punctuation (i dont like that). Use
 abbreviations only if you are sure everyone will understand them
 ("imho" "btw, "for example).

(5) Break your writing into paragraphs: screenfuls of text are
off-putting. "White space" separates your ideas, makes it
easier to quote selectively (see #9 below) and encourages
recipients to read your message in full.

 (6) If you have nothing to say, say nothing. Unless your fellow users
 are very patient, emails that just say "me too", "me neither", "I  agree"
 or (worse) "I don't know anything about this subject, but ..." are likely
 to irritate. Such messages might be better sent as a private email to
 the sender (do this by copying and pasting the private party's
 address into your mailer - most YahooGroups are set to
 reply to the whole list).

(7) Give your message a clear subject title. If you read your
 messages as a daily digest, try to refer to the subject of the
 thread to which  you are replying, rather than digest #4203,
 as appropriate.

(8) Do not quote lengthy messages or entire digests in
 your  reply. It is more annoying than you probably realize for
 users who read their messages in a daily digest, and it increases
 the time and cost of downloads for others. Similarly, a two line
 "signature" should suffice -- especially if you are frequent
 correspodent (we all know who you are! - and put your profile
 into the Yahoo Group site so we do!).

(9) Write for the lowest common denominator. Assume your reader
 is  using telnet across a 12k dial-up modem on a slow 386 or an
 Apple II. Don't use html, don't use fancy graphics and colours and
 don't assume that links are clickable. Remember that internet
 access is expensive in some parts of the world, and many
 people pay per minute.

(10) Break one of these rules rather than go against your
COMMON SENSE -- the best guide to (n)etiquette ever discovered.