Mail's Address History (Mac OS X 10.2.8 or earlier) or Previous Recipients (Mac OS X 10.3 and 10.4 or later) feature helps you automatically complete email addresses as you type them. Learn how to tell Mail to forget an address.
Mac OS X 10.2, 10.3, 10.4 or later
Mac OS X 10.1.5
Open Address Book, locate and select the address(es) to remove, and click Delete.