Use email portal
Instead of nautilus-sendto, so that it works inside and outside a sandbox. See https://gitlab.gnome.org/GNOME/yelp/blob/master/libyelp/yelp-view.c#L123
Example usage of the D-Bus API: https://github.com/flatpak/flatpak-xdg-utils/blob/master/src/xdg-email.c