Could you fix this by changing/specifying which email client to use within your browser settings?
To do this in IE 7, select "Tools"->"Internet Options" click the "Programs" tab, then select which email client to use from the list.
(Not sure if this will immediately work with whatever your web email client is, but your email provider might offer a browser plugin that will enable this.)
But this is totally a shot in the dark...I'm just assuming that the mailto protocol is ultimately being processed through the browser.
Does anyone know if this will work? Or can anyone clarify how the submit button works on the back end?