Is the A330F main deck cargo compartment pressurized? If not, what is the purpose of the structural cargo access door? I know there is a related question already asking if cargo holds in general are pressurized.

My question is specifically about the A330-243F in the below photograph. That looks like a pressure withstanding door/bulkhead dividing the fore crew compartment from the aft cargo compartment. Is that correct, and does it mean the aft cargo compartment is not pressurized? It's also possible both are pressurized and it's an airtight door for fire/fumes control.


Source 

As mentioned in the linked question, it's likely both compartments are pressurized because of the efficient shape of the pressure vessel and so that the same pressurization system can be used across both passenger and cargo variants.
You're correct- for cargo aircraft derived from passenger planes, the cabin is pressurized. This reduces the complexity and certification requirements. Also, this allows for easy conversion too. The thick door may be due to two reasons:


To improve load carrying/transferring capacity- airbus talks about 


  ... the aircraft’s reinforced fuselage and doors, which increase shear and bending/running loads..


also, the thick door will help protecting the crew should the cargo move forward in flight.
To act like an airtight seal between the cabin and cockpit, so that it prevents smoke ingress into cockpit and to enable the crew to extinguish fire. FAA Class E (for cargo aircraft) requires isoaltion between the cargo compartment and cockpit.


Another thing to note is that even a number of military 'cargo' aircraft have pressurized cabins (An-124, I think is among  exceptions), as they are also used in Medivac and causality evacuation roles- these require the cabin pressure to be set higher than normal.