I have 2 email addresses for one of my friends. When I reply to his emails hotmail defaults to the wrong older address. How can I prevent this?

He used to be one of my contacts, but ever since all my contacts received spam about enlargement etc I have deleted all contacts.  The only other thing I can add is that when I originate a new email to him, hotmail's auto-complete of text for names offers both names, so it's easy to remember and choose the right one. The problem is when I just hit 'reply' and forget to check which name.  His emails obviously come from the new correct address, so it's curious that hotmail picks up an older address he hasn't used for over 2 years.