Mail 4.6 (under 10.6.8) automatically grabs old addresses no longer in address book (5.0.3). This is a constant problem forcing a lot of vigilance to avoid delivery failures. How do I permanently purge out-of-date addresses that do not show up in address book?
With Mail open, go to the Window Menu at the top of your screen and select Previous Recipients. A new window will open. Now you can scroll down the list and just click on the email address you want removed and click on the remove from list button.
