How do I remove an email address that Mail keeps suggesting I use?
Answer
Note - This is known also as clearing entries from the "auto complete" feature in Mail
When you are using Mail, it sometimes tries to be helpful by suggesting you might want to use an email address you've used already. In the image below, I am about to email Frank but you can see that Mail is also trying to suggest that I might want to contact Fred....
Fred isn't someone I contact anymore, so I don't want his name appearing (this is also useful if an alternate/old address keeps appearing here and you need that removed) so go to the "Window" menu in Mail and choose "Previous Recipients"...
In the window that appears, select any name/s you want to remove and click on the "Remove From List" button. If it is an important email address that you want kept, use the other "Add to Address Book" button to add the name into your address book...
Credit to the Apple discussion forums for this tip : http://discussions.apple.com/thread.jspa?messageID=9107911