Email-Security-Getting email with two Senders

In a mail from a friend i see a strange adress ontop-where one can add it to the adress-book.
have seen a similar "problem" when checking my friends skype-account. he has only one email-adress.
it was coming in at an email-provider where he is not sending to normally. not coming in outlook on my winxp-pc.
Will now check if its a real friend. yes there was a big confusion in win7-email(windows live...) where he had opened an account with the wrong email-adress. already deleted. Win7