From the book: 
The Lock and Key Library; Classic Mystery and Detective Stories
Edited by Julian Hawthorne 

Published by The Review of Reviews Company - New York - 1909

On June 8, 1872,  law was approved making it a penal offense to use the mails for the purpose of defrauding others, whether residing within or outside of the United States. 

The postmaster-general was also authorized to forbid the payment of postal money orders to persons engaged in fraudulent lotteries, gift enterprises, and other schemes for swindling the public, and to instruct postmasters to return to the writers, with the word "fraudulent" written or stamped on the outside, all registered letters directed to such persons or firms. 

Prior to the enactment of this law, the most wholesale and barefaced operations were conducted by professional cheats, mainly through the facilities afforded by the mails, with almost absolute impunity. Letters addressed to bogus firms were indeed forwarded from the offices of delivery to the department as "fictitious" and "undeliverable," and many colluding postmasters were decapitated. Such petty measures of warfare served merely to annoy the vampires and to whet their diabolical ingenuity for the contrivance of new devices. 

Since the law of 1872 went into effect, however, the scoundrels have been compelled to travel a thorny road. Scores of arrests have been made, and in many cases the criminals have been sentenced to the penitentiary.