Hello,
For a N-channel MOSFET the intrinsic diode will only "catch" negative voltage excursions (below ground potential). However, when the solenoid turns off, the energy within its inductance will drive the drain of the MOSFET positive (above the supply potential). The intrinsic diode will do nothing in this instance.
The connection of a diode in parallel with the coil of the solenoid, as was previously mentioned, would require that the anode of the diode be connected to the drain, and the cathode to the positive supply.
Regards,
Mac