Title 18 U.S. Code § 1341, Frauds and Swindles, lays out the offense of mail fraud. Mail fraud is committed when an act of fraud is perpetrated through the use of the U.S. Postal Service or any interstate mail carrier. Examples of mail fraud include sending false information through the mail in order to secure government benefits, using false addresses and false identifications to defraud public and private entities, misrepresentation in advertising such that a person pays for one thing only to receive a much lower value item, and get-rich-quick schemes, among many other scenarios.
The federal government will prosecute those it believes to be involved with the attempt or conspiracy to commit mail fraud, so charges can be brought without a single letter having been sent and can ensnare a number of people with varying levels of knowledge and responsibility.