Traditionally, many mailing lists set the Reply-to header field to the mailing list address, so that replies would go back to the mailing list by default. For various reasons, that has fallen out of favor. Nowadays the message headers of mailing list messages usually indicate the list address in a separate header field, and mailing-list aware email clients provide a “reply to list” action. Unfortunately, too few email clients are mailing-list aware in that way. “Reply all” is the fallback, but make sure to remove the non-mailing list address, or else the original sender will receive two copies of your reply (one through the mailing list and one directly), which can be annoying.