-- 
Sébastien BLAISOT
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:
1. Keep current behavior.
2. If To: and Reply-To: are the same and From: is different - add From:
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.