It is not difficult for a person to send an email to another that appears to come from a third party. Spammers do it all the time, right? Of course, with a basic amount of due diligence, the receiver can figure out the ruse. But, if you were to send an email to my wife or my father-in-law that appears to come from a person they know and is more-or-less "in character" for that person, it's entirely possible they'd be fooled.
Like, if my aunt was arriving on a flight on Thursday and my father-in-law was picking her up, I could easily fake an email from her telling him that my flight was changed to Friday and he'd buy it. However, if I tried to send an email to my father-in-law that appeared to come from my aunt telling him that he had to send her $5,000 right away, it's less likely to work.