It is a system that I knew there decades when I had a business. In France, it was called the "Nigerian Connection" because the letters were from Nigeria for most. Letters arrived by mail, envelopes were recognizable because the envelope, there was a beautiful stamp, depicting flowers and exotic butterflies. The letters at the time typed, original, not copies, good quality work, were all more or less the same scenario. The sender (or sending) was as the person entitled to the estate of a person with an important post in government, who in his lifetime he could collect large sums in the context of public procurement. But these are (several hundred million) were deposited in a bank and only one foreign company could have it routed to an account in France account. So the person demanded that ensures the contest in exchange for a percentage, which, on paper, was a big bundle of millions of dollars! In reality, those who had the misfortune to meet, within days of receiving a new message, but with a header more or less official tax service of the country saying that the operation could be done without problem, but there was a small fee of 0.3% payable. Modest rate of course, but on the total, gave a very high amount. Most people, sniffing the scam, do not give away. They were beginning to receive documents that look like official papers that threatened legal proceedings to French courts to recover "these taxes." There has even had phone or on the street threats, trying to convince them to pay.
At the time there was some investment in time and money to the scammers, having to retrieve mailing lists of companies in the Chambers of Commerce, typing letters to the machine, buy stamps and envelopes.
Today, with the Internet, which was a semi-professional activity, may be made by any person, individually and with a minimal investment, if not zero. Just register on dating sites and target types of individuals likely to be handled, including married, gay or straight, because everything is used. Just get them to talk the victim to get photos, names and addresses, marital status, etc. Then it moves to Phase 2 where the blackmail begins with threats to reveal the spouse eg virtual infidelities (as it does not exceed this stage) by sending copies of messaging or even by going directly to domicile. I knew someone who had problems in her marriage due to this stuff, letters having arrived home.