There are three main mailing lists that relate directly to X11 on OS-X.
X11-Users
x11-users is hosted by lists.apple.com as a forum for users to interact with developers of X11 on OS-X.
XQuartz-Dev
xquartz-dev is hosted by lists.macosforge.org as a place for developers to discuss development topics in more detail than would be appropriate for the x11-users list. Submit patches and discuss development in this list.
XQuartz-Changes
xquartz-changes tracks changes to our repositories.