Recently, we have experienced emails sent to Yahoo are being deferred. Deferred messages are those that are in a temporary state of neither being delivered nor rejected. Yahoo has started "greylisting or extremely deprioritizing" almost every email that goes through their mail server. Greylisting - this is a way of attempting to block SPAM. Yahoo mail denies the first of every delivery attempt of an email (451 Message temporarily deferred - 4.16.50) from deprioritized mail servers. They assume that SPAMers don't try sending the same email twice, so they put the IP address of that email server on a list and then if the delivery is retried within a short amount of time, they assume that it is a good email.
For more info on how an email is being deferred, please visit this link
Other source: Yahoo Mail Servers Rejecting Email