By definition, discussion mailing lists are intended to foster discussion amongst the subscribers. For this reason, listowners will generally request that the list be configured so that replies be directed back to the entire list instead of to just the original author.
This is done by adding a "Reply-To" header to every message, which points back to the list address. Thus regardless of the sender, when the recipient hits reply, that reply goes to the entire list.
Recipients can always edit the To: header on that reply to send it only to the original sender if they wish, but few will do that.
One potential problem with using Reply-To in this way is that a subscriber's defective out-of-office autoresponder might trigger a mail loop. This is mitigated for DataBack lists by limiting the maximum number of posts each day from any single subscriber (generally 12 max). If a loop does get started for some reason, subscribers then receive at most 12 copies.
We can instead configure a list so that replies go only to the original author. In that case, each post intended for the list must be specifically addressed to the list, including replies.