Gary Rosenzweig writes for MacMost:

When you type and email address in Mail, you will get some suggestions that include your Contacts and also a list of previous recipients. Some of these addresses may be old or ones you don’t wish to see anymore.

You can delete them using the Previous Recipients list in Mail.

Full article here.

MacDailyNews Take: Cleaning “Previous Recipients” can be very satisfying, especially if you’ve never don it before!