Remove obsolete 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 so Mail stops suggesting them.
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 a card doesn’t exist yet and you want to keep the address in Contacts, 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.
Your Previous Recipients list is available on your other Mac computers (with OS X 10.8 or later), iOS devices (with iOS 6 or later), and iPadOS devices where you’re signed in with the same Apple ID and Contacts is turned on in iCloud settings.