I had an email the other day which looked as if it came from a company with whom I had done business, headed : "Your outstanding invoice with us." In the text it said something like : "Your in owing to us the amount of $437.32 and before we get legal acting you are invited to pay." Very authentic. There was also a .pdf which I didn't open.
I wrote back to them and said words to the effect of : "Disappear and sue me ........" No further response.