You may recall that Yahoo implemented a "p=reject" DMARC policy in April, 2014 for their primary yahoo.com domain name. (And AOL did the same for aol.com shortly after.) This changed the email landscape significantly. Among other things, email forwarding, discussion groups, and spam were all impacted, for better or for worse.
Today, Yahoo announced on the DMARC-Discuss mailing list that they will be similarly implementing a "p=reject" DMARC policy for their ymail.com and rocketmail.com domains on November 2, 2015.
The domains ymail.com and rocketmail.com are alternate domains that Yahoo! Mail users can use when creating an account, thus, they are pretty much equal to yahoo.com when you consider what people use them for or what kinds of traffic you would typically see them used for.
A Yahoo representative also explained that "[in] the coming quarters you can expect Yahoo to publish similar policies for other Yahoo owned and operated domains, including international Yahoo domains (e.g. yahoo.ca), Yahoo Groups, Flickr and Tumblr."
If you run mailing lists or email forwarding, and you've already updated your software to appropriately handle domains with a DMARC "p=reject" policy, you probably don't have to do anything new here, assuming you didn't just hard code your software to special case aol.com and yahoo.com.