Guaranteed to be as crap as everything else they do. I use a Yahoo mail account for spam mail. Such a shockingly bad UI, it's not even worthy of my spam. I would say it's up there in the worst UI's of all time list. Oh and the Yahoo iOS mail app, I don't think it's ever worked for me. Two clicks (if I'm lucky) and it crashes, or doesn't let me sign in for some stupid reason.
7/28/15 at 3:24pm