The IMAP account name used not to be a valid email address on this system, so replies are beimg sent to the default email address for your domain.

To fix this go to Options at the top of the webmail page, then click "Personal information" and make sure your email address is correctly set. You might like to set your name correctly while you are here too. You don't have to set reply-to if you set your email correctly, but you may if you wish.