Whenever someone sends you a message in Mail or you send someone a message, that person's name and email address are added to your Previous Recipients list in Mail. With Mac OS X 10.4 and 10.5, you can easily add any of those recipients to your contacts list in Address Book. Go to the Window menu in Mail and select Previous Recipients. Highlight all of the names in the list that you want to add to Address Book and click the Add to Address Book button. All of the selected contacts will now be in Address Book.

previous_recipients_to_ab