The Yahoo bounce problem has been resolved. There were erroneous ‘554: this user does not have a yahoo.com account’ between March 14 and March 16. If you attempted to send mail and received this bounce during that time you can reactivate the address in your database. Most ESPs should be able to help you with this.
Moving forward, though, these bounces are valid and addresses should be removed from your list according to standard data hygiene processes.