Those 1,700 are easy to find though, just need to dig a bunch of domain names and you'll find plenty vulnerable ones that you can spoof.

Yahoo Mail has a market share on the order of 3%. So a black hat could then target a decent chunk of users with @yahoo addresses specifically.

Has anyone heard of this being exploited in the wild? Would be interesting to find out whether there are some reputable domains among the 1.7k vulnerable ones.