Spies come in many shapes and sizes, but the best ones are invisible.
When the General Post Office was established in England in 1657 (a tumultuous moment, to be sure), it had the express mission not only to promote commerce and transmit dispatches (as it still does), but also to undertake what we would now call surveillance.
What’s more, lots of people knew that their letters were being read (either with or without a warrant) — and complained about this fact in, well, their letters. (See also: passive-aggressive.) Many tried to thwart unwanted readers by encrypting their messages, with varying degrees of commitment. (For state intelligence, writers painstakingly deployed sophisticated encryption techniques; for gossip about well-known figures, they might merely have used code names.)
But encryption was not always effective. For missives thought to contain crucial intelligence, the state could call upon talented cryptanalysts to decipher key messages.
Once a secret got out, however, it only remained valuable if its escape remained secret. Usually, the revelation was itself revealed. Recipients could tell when their letters had been opened and read, since those dispatches would later arrive with torn pages or broken seals. (Letters were frequently sealed closed with wax, into which a design was stamped.)
Not all victims of unauthorized letter-opening were aware that certain dispatches had been intercepted, however — openers covered their tracks when they wanted correspondents to think that their secrets were still safe.
During the 18th century, England enlisted skilled engravers, who set about clandestinely forging seals so that, when previously-opened dispatches arrived, they would appear never to have been touched.
This resealing practice might be considered the 18th-century equivalent of marking a hacked email message as unread — but it required a good deal more craftsmanship, not to mention secrecy, to carry out.
Ellis, K. (1958). The Post Office in the Eighteenth Century: A Study in Administrative History. New York: Oxford University Press.