Here's a screenshot of a qpsmtpd processes on one of the servers. It was actually a few weeks ago but didn't get around to posting it until now. It looks like all the Yahoo contacting servers went out of control for a couple of hours on that morning.

This screenshot shows that Yahoo was contacting one of our servers more than 20 times at this particular moment. You'd see this in the past, but now servers that act this way are usually blocked or blacklisted because it's doubtful that the sending servers have anything worth while to say and is most certainly spam. Accordingly, I never found out as the message never reached anyone's inbox. I suppose the spam filters were doing their job.