The whole thing is obviously a set up. There is no one in the "mailbox" at the time the man shoots into it.
BTW, people can be killed by blanks used improperly.
[Jon-Erik] Hexum is most famous not due to any of his roles but rather the bizarre nature of his death. Hexum died after shooting himself in the head with a prop gun loaded with blanks on the set of the CBS series Cover Up, a program about a pair of fashion photographers/models who were actually secret agents. Hexum, who played a weapons expert, was said to constantly be playing with the guns as if they were toys and once angered Jennifer O'Neill so much that she chastised him on-set for his carelessness. [3]
On October 12, 1984, after finishing a scene in which he fired several blank rounds from a .44 Magnum revolver, Hexum jokingly put the gun up to his temple and sarcastically said,
"Let's see if I've got one for me."
Hexum apparently did not realize that blanks use paper or plastic wadding to seal gun powder into the shell, and that this wadding is propelled out of the barrel of the gun with enough force to cause severe injury or death if the weapon is fired at point-blank range. The paper wadding in the blank that Hexum discharged struck him in the temple with enough force to propel a quarter-sized piece of his skull into his brain. According to a crew member on the set:
"John smiled and pulled the trigger. There was a loud bang and a bright flash, then black smoke. Jon screamed in agony, then looked kind of amazed as he slumped back onto the bed with blood streaming from a severe head wound. It was horrible."
Hexum's assistant ran to him and wrapped his head in a towel. An ambulance was called, but before it could arrive, Hexum slipped into a coma, prompting crew members to carry him to one of the studio's station wagons and drive him to Beverly Hills Medical Center. Hexum went into surgery as his family and girlfriend, actress Elizabeth Daily, were notified of his condition. Initially he was listed as being in "serious" condition, but after five hours of surgery, doctors changed the condition to "critical." Hexum was given a feeding tube and respirator, and lingered for six days before doctors pronounced him brain dead. With his mother's permission, Hexum was flown to Las Vegas and taken off life support so that his organs could be donated.
http://en.wikipedia.org/wiki/Jon-Erik_Hexum