Mailto links should be redirected back into Roundcube so that it composes and sends the email itself rather than the default mail client on the system. Perhaps make it a preference option, but it makes sense to me.
Thanks.
Unless I'm mistaken, you'll run into this problem with any web-based mail client. This won't be a function of roundcubemail to "fix" where the mailto is going.
It will depend on where and how each OS points to the mail client. In the case of Windows it'll be a registry hack.
Thanks, dan