On Posting
Before asking a question, first make an effort to find the answer to your question. A lot of questions have been asked and answered before, so please spend some time trying to solve the problem on your own (e.g., search the web, search these web pages, etc.), and show us that you did so when you ask your question.
When asking, (1) be details, and (2) demonstrate that you have made an effort, e.g., "I am having trouble frobbing the foo. I searched the web and only found information regarding how to frob a bar, but that seems unrelated." Provide as many relevant details as possible reproducing them as exactly as possible.
This document may help you understand some developers attitudes and social norms.
List etiquette is to cc the sender and anyone actively involved in the discussion. Some people don't like this and they set their reply-to header appropriately. Respect this.
It is rarely appropriate to cc multiple lists. Sometimes it is. In that case, do so. If in doubt, don't; just choose the single most appropriate list.
When replying to others, please don't top-post and trim the existing text to quote only the sections you're actually replying to. See Posting style for further reading, especially Posting style#Interleaved style.
Also see these general notes about communication.
Main Lists
Most of these lists are unmoderated, and most of them are hosted on http://lists.gnu.org/.
Some of the Hurd user groups listed on community also host additional mailing lists.
bug-hurd
bug-hurd@gnu.org
http://lists.gnu.org/mailman/listinfo/bug-hurd
Technical discussion and bug reports; main development list. If you want to contribute, please meet us here.
commit-hurd
http://lists.gnu.org/mailman/listinfo/commit-hurd
Commit notices for the GNU Hurd, and other automated status updates.
hurd-devel
http://lists.gnu.org/mailman/listinfo/hurd-devel
Low-traffic developers list. This list is moderated and restricted. Subscribe to hurd-devel-readers instead.
hurd-devel-readers
http://lists.gnu.org/mailman/listinfo/hurd-devel-readers
This list is a read-only mirror of hurd-devel. In contrast to the former, everyone is free to subscribe to this read-only list.
help-hurd
help-hurd@gnu.org
http://lists.gnu.org/mailman/listinfo/help-hurd
Hurd-specific questions, for users of the Hurd. If you need help with using the Hurd, please ask on this list.
web-hurd
web-hurd@gnu.org
http://lists.gnu.org/mailman/listinfo/web-hurd
Discussion of the http://hurd.gnu.org/ web site.
l4-hurd
l4-hurd@gnu.org
http://lists.gnu.org/mailman/listinfo/l4-hurd
Discussion regarding a possible design successor to the Hurd.
debian-hurd
debian-hurd@lists.debian.org
http://lists.debian.org/debian-hurd/
Discussion around and questions regarding the Debian GNU/Hurd distribution.
gnu-system-discuss
gnu-system-discuss@gnu.org
http://lists.gnu.org/mailman/listinfo/gnu-system-discuss
Discussion about the GNU system.
libc-alpha
libc-alpha@sourceware.org
http://sourceware.org/ml/libc-alpha/
Discussion about glibc, where the Hurd's POSIX et al. interfaces are implemented, amongst a lot of other bits and pieces. This list is moderated, but on-topic development topics will generally be accepted.
Spam
GNU mailing lists used to have no spam filtering applied -- everyone was allowed to post, the lists were completely unmoderated. This resulted in getting the lists spammed very much, as you can see in the web archives. If you're interested in getting hold of (mostly) spam-free mboxes, then talk to Thomas Schwinge. Also read about the spam-rejection techniques being used today.