If he had mailed a threatening hoax letter to himself, and never told anyone about it, no harm no foul. But, he used a letter that he sent through the mail as false evidence of a false report of a crime. In other words, he used the U.S. Mail to perpetuate a hoax, and THAT is mail fraud.
Yup, pretty simple.
Skip the FBI...just hand it to Postal Inspectors. They are trained and do this type of work all the time. Once they determine a crime took place and have the supporting evidence lined up....let the federal justice system work. I’d tell Smollett you have one single chance to get a single-year sentence out of this...just come clean and admit it all. If not....prosecute for four years minimum in prison.