Delete email addresses in Mail on Mac
Mail keeps a list of your previous recipients and uses it to suggest or complete addresses for you when you send email. You can delete obsolete or incorrect addresses from the list in Mail.
In the Mail app on your Mac, choose Window > Previous Recipients.
Find the address you want to delete. You can search for it or sort the list by clicking a column header.
If you want to check the address in the Contacts app, double-click the card icon in front of the address. If you don’t see a card and want to keep the address in Contacts, just select the address, then click Add to Contacts.
Click Remove From List.
Tip: If you use Smart Addresses, you can remove a recipient from the list when you address a message by clicking the arrow in the address.
If you use iCloud Contacts, your Previous Recipients list is available on your other Mac computers (with OS X 10.8 or later) and iOS devices (with iOS 6 or later) that have iCloud Contacts turned on.