iCloud: Remove email address suggestions in Mail on iCloud.com
Mail keeps a record of the email addresses you send email to, and uses these recent recipients to suggest or complete addresses when you send new email. You can delete obsolete or incorrect recent recipient addresses so iCloud Mail stops suggesting them.
In Mail on iCloud.com, click the Compose button.
Type a recent recipient’s name or email address in the To field.
Move the pointer over the list that appears below the To field, then click the Remove button to the right of an address or group of addresses you want iCloud Mail to stop suggesting.
The list may also include email addresses from iCloud Contacts. To remove one of these, you must delete the contact.