There’s an interesting thread (“Yahoo.com hates us. Suggestions“) over at the mailop mailing list. I’ve encountered almost every behavior from Yahoo! Mail servers that is documented there. Unfortunately the mailop archives are not open to the public, so you need to subscribe first.
In our case, when we deal with Yahoo! Mail delivery problems, it is almost always a case of infected machines (sometimes even a handful) sending spam …everywhere. So whenever we observe long delays while delivering to Yahoo! Mail or many many messages waiting to be delivered, we always seek for the infected. Thanks to feedback loops that are implemented by the (really) big email hubs, we also get early warning on such matters. As a matter of fact, Yahoo! Mail also runs a feedback loop, but it requires DKIM, and since we’ve stopped using DKIM (dkim-filter crashed frequently on our systems) we rely on the rest of the loops to be kept …in the loop. It seems to be working OK so far.