A more thorough explanation:

A decoy letter is a letter that is intentionally prepared and sent to catch a criminal who has broken postal or revenue laws. The purpose of the letter is to trick the criminal into revealing their identity or location.

For example, if a postal worker suspects that someone is stealing mail, they may prepare a decoy letter with fake information and send it to an address where they suspect the thief is operating. If the thief takes the bait and steals the decoy letter, they can be caught and prosecuted for their crime.

Another example is when the government suspects that someone is evading taxes. They may send a decoy letter pretending to be a potential client or business partner, hoping to catch the tax evader in the act of committing fraud.

Overall, decoy letters are a useful tool for law enforcement to catch criminals who are breaking postal or revenue laws. They rely on deception and trickery to catch the criminal, but ultimately serve to protect the integrity of the postal and revenue systems.