Simplify the following text for a Grade 6 native English-speaking person.
Original: While officially known as the `` U.S. Postal Service , '' despite the passage of nearly 40 years the public still colloquially refer to it as the `` Post Office '' or `` U.S. Mail ''
Simplified: In the United States , it is referred to as `` the post office '' , `` the postal service '' , or just `` the mail '' .