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The advice below is based on years of experience with the Subversion mailing lists, and addresses the problems seen most frequently on those lists. It should not be taken as a complete guide to mailing list etiquette — you can find one of those on the Net pretty easily if you want one.
If you follow these conventions when posting to our mailing lists, your post is much more likely to be read and answered.
When in doubt, mail email@example.com, not firstname.lastname@example.org. There are many experienced people (including some of Subversion's maintainers) on users@ list — they may be able to answer your question, or if you think you've found a bug they can determine whether or not it is a genuine bug. You should even post to users@ when you want to suggest a new feature: many feature suggestions are ideas that have been discussed before, and someone on the email@example.com mailing list will usually be able to tell if that's the case with your suggestion.
Please do not post to dev@ as a last resort after failing to get an answer on users@. The two lists have different charters: users@ is a support forum, dev@ is a development discussion list. When a support question goes unanswered on users@, that is unfortunate, but it does not make the question appropriate for dev@.
Of course, if the mail is about a possible bug in Subversion, and got no reaction on users@, then asking on dev@ is fine — bugs are a development topic. And patches should always be sent directly to dev@.
Sometimes, when really impassioned about a topic, it's tempting to respond to every message in a mail thread. Please don't do this. Our mailing lists are already high-traffic, and following up to every message only adds to the noise.
Instead, read the entire mail thread, think carefully about what you have to say, pick a single message to reply to, and then lay out your thoughts. Occasionally it might make sense to reply to two separate messages in a thread, but only if the topics have started to diverge.
Please don't use lines longer than 72 columns. Many people use 80-column terminals to read their email. By writing your text in 72 columns, you leave room for quoting characters to be added in future replies without forcing a rewrapping of the text. The 72-column limit only applies to the prose part of your message, of course. If you're posting a patch, see the section on patches.
Some mailers do a kind of automatic line-wrapping, whereby when you're writing your mail, the display shows line breaks that aren't actually there. When the mail reaches the list, it won't have the line breaks you thought it had. If your mail editor does this, look for a setting you can tweak to make it show true line breaks.
Capitalize the first letter of each sentence, and use paragraphs. If you're showing screen output or some other sort of example, offset it so it's clearly separate from the prose. If you don't do these things, your mail will be much less readable than it could be, and many people will not bother to read it at all.
Don't start a new thread (subject) by replying to an existing post. Instead, start a fresh mail, even if that means you have to write out the list address by hand. If you reply to an existing post, your mailreader may include metadata that marks your post as a followup in that thread. Changing the Subject header is not enough to prevent this! Many mailreaders will still preserve enough metadata to put your post in the wrong thread. If this happens, not only will some people not see your post (because they're ignoring that thread), but people who are reading the thread will waste their time with your off-topic post. The safest way to avoid this is to never use "reply" to start a new topic.
(The root of the problem is really that some mail interfaces do not indicate that the message generated by the "Reply" function is different from a fresh message. If you use such a program, consider submitting an enhancement request or a patch to its developers to make it show a distinction.)
If you do need to change the Subject header while preserving the thread (perhaps because the thread has wandered into some other topic), do it by making a post under the new subject with the old subject in parenthesis, like this:
Blue asparagus | |_ Re: Blue asparagus | |_ Yellow elephants (was: Re: Blue asparagus) <-- ### switch ### | |_ Re: Yellow elephants
Please don't reflexively chide people for top-posting. "Top-posting" is the practice of putting the response text above the quoted text, instead of interleaved with it or below it. Usually, the quoted text provides essential context for understanding the response, and so top-posting is a hindrance. Sometimes, people top-post when it would have been better to inter-post or bottom-post, and others chide them for this. If you must chide, do it gently, and certainly don't bother to make an extra post just to point out a minor problem like this. There are even situations where top-posting is preferable — for example, when the response is short and general, and applies to the entirety of a long passage of quoted text. So top-posting is always a judgement call, and in any case it's not a major inconvenience even when done inappropriately.
If you came here looking for advice on how to quote, instead of advice on how to not flame people for their bad quoting habits, see http://www.netmeister.org/news/learn2quote.html (Deutsch: http://learn.to/quote).
See here for advice on how to send in a patch. Note that you can send in a patch to modify these web pages as well as to modify code; the web pages' repository URL is http://svn.apache.org/repos/asf/subversion/site/.
Please use ASCII or ISO-8859 text if possible. Don't post HTML mails, RichText mails, or other formats that might be opaque to text-only mailreaders. Regarding language: we don't have an English-only policy, but you will probably get the best results by posting in English — it is the language shared by the greatest number of list participants.