You can check the behavior by clicking Reply All button (not Reply List) on this message. Example message from our mailing list contains headers: To: dev@lists.roundcube.net Reply-To: dev@lists.roundcube.net From: original@poster.tld
So, current behavior is that you end up with one recipient dev@lists.roundcube.net in To field. Thunderbird for example, will add original@poster.tld in Cc field.
So, the problem is related to Reply-To header handling. Poll:
address to Cc field.
I prefer current solution (1). Because I think that if Reply-To is defined a senders intention is to not receive replies to his From: address, no matter if Reply, Reply-All or Reply-List is used. I however understand that "private reply" to poster is sometimes wanted.