The incident happened in front of a private house on 222nd Street in Springfield Gardens.
Police say the package was sitting on the front stoop for a number of days.
The landlord, a 73-year-old, picked up the package, and it exploded.
The package appeared to be a cylinder, similar in shape and size to a Quaker Oats container.
Police say there was no evidence of shrapnel, just black powder.
The NYPD, FDNY and EMS are on the scene.
The victim is said to have suffered second and third degree burns over 80-percent of his body, and is currently in critical condition at Nassau University Medical Center.
The U.S. Postal Service released a statement:
"This matter is currently under investigation with law enforcement and we cannot provide further comment at this time."
The victim owns the home but does not live there. It is currently rented out by two families.
The investigation is ongoing.