My Bulk folder fills up with spam. I move spam emails to my Inbox where I mark them as spam.
If Yahoo could match each email received against my address book, potential span could be identified before I have to deal with it.
Another email provider offers a feature where these emails are held until the addressee has added the new sender to their address book. If the address is not added in a period of time, the emails are deleted and are never sent to the original intended recipient.