> The RCU list is the only recipient, though. That's how "this kind of 
> list" works.

There indeed is something broken about the roundcube list, but it is not 
just because of Reply-To munging.

Example... the dovecot email list also does Reply-To munging (if you 
just click 'Reply', the reply goes to the list) - BUT - if you click 
'Reply-All', the reply goes to both the list AND the original sender.

So, the roundcube list is doing something WRONG/different that breaks 
'Reply-All' functionality.