The Mail app in Mac OS X and macOS fills in a recipient's email address as you begin typing it into the To, Cc, or Bcc fields of an email if you've used it previously or entered it in a Contacts card. If you've used more than one address for that person, Mail displays their options below the name as you type it, and you can click the one you want to use.
Remove Junk Email Addresses in Mac Mail (#1307)
However, people change their email addresses. There is a way to remove old or unwanted addresses from the Auto-Complete list in Mail. The Mail application will automatically remember all new addresses and soon the auto-complete function will be useful again.
If you want to remove auto-complete addresses, you can do so by working directly in the Previous Recipients list. To remove an email address from the auto-complete list in Mac OS X Mail or macOS Mail:
Open the Mail application in Mac OS X or macOS.
