Brand new 10.10 Yosemite mail.app can not work with Yahoo Mail?
I am sure you did all these things, but just in case...


Turn POP access in Yahoo mail through browser:
https://help.yahoo.com/kb/mail-for-desktop/SLN15404.html?impressions=true
Check you IMAP settings in Mail app:
https://help.yahoo.com/kb/imap-server-settings-yahoo-mail-sln4075.html
Check that you actually do have messages in your Inbox (through browser) :)
Check that you do not have Forwarding ON:
https://help.yahoo.com/kb/automatically-forward-emails-yahoo-mail-sln22028.html


If this didn't help, let's search further...