Because I've created a reserved area, where I've written a custom form to send the emails trought MailChimp. It permits to change the name and the email "from" fields, but it does not permit to read the replies to that emails. So I've tought to use an email client to read the replies. I've search for many WordPress plugin or similars, but I didn't found anything that worked properly. I hope that RoundCube permits me to do that... I'd like to put RoundCube into WordPress pages (and interface) because I don't like to force users to change the graphic theme and exit the WordPress theme layout and menu, also because I don't know how to permit them to return to the WordPress site