Check Your Forwarding Settings
As many email providers, Yahoo Mail allows you to forward all your emails to another mailbox. It is sometimes used by attackers to continue tracking a victim emails after they reset their password.
To check your forwarding settings, go to your Inbox, click on Settings > More Settings > Mailboxes, and click on your mailbox name. On the bottom of the page, you will see the forwarding section with email addresses :
You should not have any email address listed there unless you configured it otherwise.