Probably you were responding to a message that I sent to the mailing list. My message contained a Mail-Followup-To field showing the mailing-list address. If your MUA doesn't understand Mail-Followup-To, ask your MUA implementor to read http://cr.yp.to/proto/replyto.html.
If you're running qmail 1.03, you can generate Mail-Followup-To fields in your own mailing-list messages. Simply set $QMAILMFTFILE to $HOME/.lists, where $HOME/.lists shows all the mailing lists you've subscribed to.
I'm generating these bounces with the experimental iftocc program in mess822. If you don't think your message should have been bounced, please let me know.