So I have a Yahoo account and just like other accounts, I get daily emails on it. The only issue is that even if I delete the mail, it pops right back up when a new mail on Yahoo arrives, or after a sync occurs. So unless I go online and actually delete the emails from trash, they always keep popping back on as unread messages on my phone.
Try this and if this doesn't work is there a way for you to install the yahoo email app? One of the reasons I saw was that yahoo changed their email settings so it could not be used with anything other than yahoo email app. That is what I use on my phone.
I didn't know they allowed tapping into Yahoo mail via IMAP and POP3! I have another hack which I setup years ago, so all my Yahoo mail goes to Gmail. Once in a while I go to Yahoo and delete AAAAAAllllllllllllllllll the emails there. Pretty simple, really.