Yes according to the USPS postal regulations it is illegal to block someone else's mailbox which interferes with the postal employees ability to deliver the mail.
You could call the police non emergency phone number and have them come out and take care of the issue and they can usually give the car owner a ticket for blocking in front of your mail box.
If the police do not do anything about it then you need to contact the USPS postal inspector in your area and tell the postal inspector that a car or other vehicle is always blocking your mail box and you cannot get your mail.
They can then proceed with the law and have the car removed through legal means.
But first I would contact the police if you cannot get the owner of the car to move the car first.