This is the command to set the Kopano Deskapp app to be the default app for mailto: links in an Linux xdg-compatible desktop environment. It assumes that a kopano-deskapp_mailto.desktop file exists :
xdg-mime default /usr/share/applications/kopano-deskapp_mailto.desktop 'x-scheme-handler/mailto'
Use the following to query what it is currently set to:
xdg-mime query default 'x-scheme-handler/mailto'