Great summary via krebsonsecurity.com:
"BEC or CEO Fraud schemes are an increasingly common and costly form of cybercrime. According to the FBI, thieves stole nearly $750 million in such scams from more than 7,000 victim companies in the U.S. between October 2013 and August 2015.
CEO fraud usually begins with the thieves either phishing an executive and gaining access to that individual’s inbox, or emailing employees from a look-alike domain name that is one or two letters off from the target company’s true domain name. For example, if the target company’s domain was “example.com” the thieves might register “examp1e.com” (substituting the letter “L” for the numeral 1) or “example.co,” and send messages from that domain.
In these cases, the fraudsters will forge the sender’s email address displayed to the recipient, so that the email appears to be coming from example.com. In all cases, however, the “reply-to” address is the spoofed domain (e.g. examp1e.com), ensuring that any replies are sent to the fraudster.
On the surface, business email compromise scams may seem unsophisticated relative to moneymaking schemes that involve complex malicious software, such as Dyre and ZeuS. But in many ways, the BEC attack is more versatile and adept at sidestepping basic security strategies used by banks and their customers to minimize risks associated with account takeovers. In traditional phishing scams, the attackers interact with the victim’s bank directly, but in the BEC scam the crooks trick the victim into doing that for them.
The FBI urges businesses to adopt two-step or two-factor authentication for email, where available, and/or to establish other communication channels — such as telephone calls — to verify significant transactions. Businesses are also advised to exercise restraint when publishing information about employee activities on their Web sites or through social media."