Sextortion is one particular kind of extortion that is defined by the Cambridge dictionary as "The practice of forcing someone to do something by threatening to publish sexual information about them."
So, it's a form of social engineering that uses different pressures than those usually seen in business email. Of course, that difference which makes it stand out may be the reason that it is being used as a technique in the first place.
A sextortionist is just a scammer who uses these not-so-veiled exposure threats to pressure victims into paying a ransom.
Typically, these scammers come out of the woodwork after a major site breach has occurred with account information exfiltrated. That's because these criminals are the ones that will purchase the account information that shows up on black hat websites after a breach has happened. They want the account information to try an unauthorized login to some other site than the one that has been breached.
One might think that the account information from one site is specific to that site. Well, it is theoretically; but people reuse passwords used on one site when logging into another. This reuse of passwords has been fairly well documented over the years, and Cofense recently issued a report that demonstrates how this insecure behavior is assumed and abused by sextortionists.
As they put it, "Why is the threat actor able to validate the account information? It's common for people to reuse the same password across multiple sites. It's just easier. Many organizations have defaulted to using your email address as your username -- again, it's simple to remember. But convenience comes at a price. Easy credentials make it too easy to compromise accounts."
They put some numbers to the scale of the problem when they also say that, "To date, we've identified over 200 million unique compromised accounts (a number that could rise significantly) and analyzed 7,854,099 sextortion emails."
They are maintaining a database at cofense[.]com/sextortion that they say can determine if an address or domain is on the target list that they have derived from all this analysis. No claims for accuracy or completeness are made about the database by them, but Cofense did find a "for rent" botnet in June 2019 that had been used primarily to send sextortion emails. They say they are monitoring this botnet and will update the target list when it becomes necessary to do so.
They also say that, "this botnet IS NOT currently infecting computers to acquire new data sets (à la Emotet). It's just recycling email addresses acquired through various means over time."
In the meantime, they advise that, "If a sextortion email is received, we recommend that you do not respond to the email or pay the ransom." Indeed, they counsel to just delete it but it's possible that it may be needed at a later time for forensic purposes.
— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.