A few days ago, Yahoo updated their DMARC policy setting to "p=reject." What this means is, mail containing a Yahoo from address is basically no longer considered legitimate if it doesn't contain an authentication signature or if it didn't come from properly identified Yahoo infrastructure. (I'm oversimplifying things there, but bear with me; I think it's close enough for this discussion. Read more about it over at Word to the Wise.)
This effectively restricts Yahoo Mail users so that they can only send from their Yahoo email address when using the Yahoo Mail web user interface. For a big segment of regular joes, this may not ever be an issue. But for some people, this is a profoundly significant new restriction on what you can do with a Yahoo email address. Indeed, this change "brings the pain" for some, as Andrew Barrett explains over on the E-mail Skinny blog.