In Yahoo Mail, it happens that, some mails are delivered very late and it gets buried deep under the previously received mails. The count on the Inbox or the other folder says there are unread mails but usually an endless scrolling reveals that there is a mail from an unreliable server or a Spam.
So how to locate and sort unread emails in Yahoo Mail?
To sort the mails in Inbox or any folder in Yahoo Mail by Unread Messages,
Click View (This is the last button in the bar which has Print, More Actions etc) -> Then Click Sort by Unread.
Now all the unread messages will be displayed first. After reading, if you want you can change the setting to the default (Sort by Date), which sorts the mail by the arrival time.