'Reply-To'

Hi! I just started using SmartList yesterday. I've got "Reply-To: $listaddr" in my rc.custom file but it doesn't work for all subscribers. When I hit reply to some of the members' messages, the reply goes to the sender. I don't have this problem myself - I get the list address when I hit reply to my own messages. Does this have something to do with the way the various mail programs are set up? Kind regards, Anne-Marie Oualit Copenhagen, Denmark