The detectDE function in /usr/bin/xdg-email checks for $DESKTOP_SESSION being "LXDE".
In Lubuntu, this check fails, because $DESKTOP_SESSION is "Lubuntu" and $XDG_CURRENT_DESKTOP is "LXDE".
A patch to test $XDG_CURRENT_DESKTOP seems appropriate and is under construction.
Once this works, a further small patch to xdg-email may be required to have it launch an appropriate client in LXDE...
The detectDE function in /usr/bin/xdg-email checks for $DESKTOP_SESSION being "LXDE".
In Lubuntu, this check fails, because $DESKTOP_SESSION is "Lubuntu" and $XDG_CURRENT_ DESKTOP is "LXDE".
A patch to test $XDG_CURRENT_ DESKTOP seems appropriate and is under construction.
Once this works, a further small patch to xdg-email may be required to have it launch an appropriate client in LXDE...