One scenario where this night not be far-fetched is when such mails are sent to the accounts payable department of large companies. The people are not going to call a line manager everytime a payment request comes through email, especially if the dollar amount is small and didn't require pre-approval.

I remember even Google had fallen prey to such a scam where they were paying somebody even though no work was done. Admittedly, that case involved fictitious invoices. However, the principle remains the same.