Great question from a client today and intelligent feedback worth your knowing ...
Client Question: "A friend received an email from what appeared to be me but the email address was different so they sent me an email asking if it was from me. It wasn't. In other words it looked like Joe Smith (This email address is being protected from spambots. You need JavaScript enabled to view it.). That's not my email address and I want to know how I can stop this and how it happened."