We have noticed delivery delays to yahoo.com freemail service. ExchangeDefender will continue to attempt message delivery for up to 5 days with a delivery attempt being made every 20 minutes. If yahoo.com has not been able to accept the message in 4 hours, sender will receive a notification email stating the delivery delay.
Yahoo.com uses greylisting and tempfail process when they are having issues. We will continue making delivery attempts and will stay in contact with Yahoo as they work on this.