Was NewHouse previously unoccupied? If so, you may find that when the car's owners realize someone is living there now, they will be more mindful of where they park. Also, has this happened more than once? First, I'd wait and see what happens - and if you happen to see the car's owner casually mention that it is causing a problem if you continue to not get mail (if you want to know quickly, you could always send yourself a letter - or postcard since they are cheaper to mail).
While you are waiting, I also suggest looking up the parking regulations for your new area. It's a good idea anyway, because you will want to know for yourself/your guests what the rules are. And what you find out can help guide you in how to proceed if this becomes an actual Issue. For example, in my area you can't park within 5ft of a driveway, so here it would be absolutely reasonable to escalate to "Neighbor, you need to stop parking so close to my driveway and blocking my mailbox" if they ignored a more passive first request like, "Hey neighbor, when you park in front of my box, I'm unable to get mail delivery. Would it be possible to park a few feet back?". In an area where the car is parked legally, if the first request is ignored talking to the post office about your mail delivery may make more sense.