Dick Sheaff from The Ephemera Society of America shows us that junk mail isn't a modern day phenomena. The same get-rich-quick schemes that exists today, existed back in the days, but then they were delivered by mail with the help of the local mailman.
The mid- to late-nineteenth century saw the beginnings of advertising agencies (most, at first, simply placing newspaper ads for their clients), of compiled mailing lists and pre-printed labels…and of junk mail fishing for fresh, new contacts. For a variety of reasons, the first junk mail (targeted mail, generic mail…take your pick) went to and through local postmasters. Small town postmasters knew anybody and everybody in town, knew their businesses, knew their interests, knew their foibles. Much such mail was addressed directly to the postmaster, asking him to pass it along to someone in town likely to be interested in the product. Other mail, addressed to "The Leading (Teacher / Grocer / Doctor / you name it)" in town also bore a side note to the postmaster asking him to re-direct it to somebody else likely to be interested, if appropriate.
Postmasters in those days were allowed to send and receive Post Office business mail for free; many a PM took advantage by engaging in some other occupation on the side using the free mail privilege. Not really kosher, but Washington largely looked the other way. Knowing this, some early junk mailers made it clear on the outer envelope that the PM was welcome to sign up as a local company agent and do some business with and for them.
"…hand this letter over to any man who wants to make $5,000":
Via Neatorama.