IF........you are using Outlook Express and under Tools>>Options>>Send you have checked off "automatically put people I reply to in my address book", well, might that explain it?
I THINK I fixed it. There is a place in the options menu which asks if you want to auto enter the sent emails into your address book .
I emailed using the "reply" button and all the email addresses which my friend had emailed, were automatically placed into my address book.
I think!!!!!!!! Joe