Some of my clients have been asking me how to get rid of those pesky addresses that seem to come up as possible addresses for what you are typing. Sometimes these are not people in your address book. Here is how to find them and if necessary, delete them. 

1. Open Mail

2. Go to the Window Menu

3. Go to the list of Previous Recipients.

4. Click on a name. If someone is in your address book they will have a little icon beside their name. 

5. Click Remove Address if you don't want them listed here anymore (If they are in the Address Book, it won't delete them from there, just from this list) or Click on Add to Address Book if you want to add them to the Address Book.  The Mail.app autofill feature pulls from the Address Book and the list of Previous Recipients in Mail.