Would you go to a job interview in your training clothes? Would you send your CV with handwritten corrections? If you answer those questions with no, you might also want to write professional e-mails. The Free Software community is a tough environment when it comes to e-mail usage. This short guide tries to help you that the community perceive you as a professional communicator.
Citation As a general rule: Do not cite the whole e-mail again, neither above your message, below or in the middle. Cite only the parts which are necessary. Use inline replying (for examples see Wikipedia's article on posting style) and trim messages if possible.
Subject Choose a good meaningful subject line. "e-mail", "help", "hello", or "questions" are not good subjects ;). When the topic of an e-mail changes, it helps to change the subject, too. Often it is beneficial to separate threads into different threads with different subjects.
Line break of e-mails Should be around 72 characters. Nobody will kill you if it is 70 or 74 or even 76. But a lot of people will get angry if you do not have a line break at all.
E-Mail signature Keep it small and simple. Signatures longer than five lines should be avoided. The separator for the signature is "-- " (minus minus blank) and then line break. The blank is important as many e-mail programs then know that it is a signature.
Mailing lists Use list-reply. It is not necessary to include the sender in To: or Cc: if he is subscribed. If the e-mail programs are configured correctly the sender will be Cc'ed if he is not subscribed or wishes to be Cc'ed.
Forwarding e-mails When you forward e-mails try to give a short summary of the e-mail. Forwarding a huge e-mail thread to a list with only "FYI" will make you no friends.
Short link Short links/tiny urls are good for microblogging, and maybe printed material where you do not have a lot of space. In e-mails you should not make use of them, but use a URL which is meaningful.
General remark If you write an e-mail always think about the reader(s). More people will be able to easily read your message if you present the information in a good form.