Yahoo sent my email to someone I don't know; not on my TO: list. How does this happen and how to stop it?

I sent an email to my family. I received a reply from an unknown person. How does this happen?
Yahoo sent my email to someone I don't know; not on my TO: list. How does this happen and how to stop it?
I've been getting failed email notices that my avatar name has sent a email to an unknown company. It says not a valid address. It comes in my inbox from Q/A. I can't figure that one out. In your case if you don't know the address you can block it, or spam it. I can't do either with the ones I get. These are probably phishing
Reply:I dont know, but i keep getting mail from people to someone using the same email address as mine, %26amp; i have no idea who the person is (its really not me).


I have wondered how they could possibly have the same email address, as my tag is very unusual.