Since about 6:00 am eastern this morning, Thursday, August 25, Yahoo has been deferring delivery attempts from almost everyone ESP or mail platform I'm hearing from. My guess is that there's a spam filter update issue or system capacity issue over at Yahoo. As far as we can see, almost all inbound mail is affected. Inbound connections are timing out, or giving unexpected TS01 errors, or giving "temporarily deferred" errors.
Stay tuned, I'm sure the good folks at Yahoo are on it and will address the issue as soon as possible.