Murray is dead on in his post. virus writers in an effort to make thier viruses more succesful (spread more) started the spoofing the sent address so that the actual person who is infected, doesn't get notifed that he is infected and can continue to spread the virus.
Many ISPs no longer send out such warnings because of the numberous false alarms and problems it caused them.
I am in a difficult situation. I see every virus that comes in our email (I work at a college) I turned off notifications to senders.
However, our instructors accept assignements via email and they are time sensitive. So every day I scan the list of viruses and if I see a word or excel macro virus, I notify the sender manually. That way they can't claim that they sent in their assignement and the instructor didn't get it. (they are not only responsible for doing thier assigment, but they are responsible for providing it in a virus free form.