Given that most of us receive more and more email messages, a key feature of an email service to me is whether it supports filter rules. Yahoo Mail needs to configure rules based on at least sender address or subject, then disposition them (delete or put into a folder). This should apply to the web UI first and foremost, and if so the mobile app would automatically benefit from it. For comparison, see Outlook or GMail rules.
Barry Meyer commented
I don't understand. The only problem that I have with filters is the problem of moving filters up or down in priority. The arrows should be locked so that we can see the current order of the filters. I'm sure in testing, only two or three filters were used. However, when there are enough filters to roll off the page, the tool becomes cumbersome.
I agree. This was in the older version and now it's been removed. Why would they do that!