Be brief without being overly terse. When replying to a message,
include enough original material to be understood but no more. It
is extremely bad form to simply reply to a message by including
all the previous message: edit out all the irrelevant material.
This is made literally impossible by some email clients and some webmail systems. Some interfaces insist on only allow "top-posting" replies, putting your new text at the top, and quoting the previous email in its entirety. It drives me nuts.I always quote the portion to which I'm replying, and add my reply below that, and recently a reply came back saying - "I like the way you reply, I'm going to reply in the same style!"
Now I can quote an RFC.