here is how i set mine up and it works, i just have to have wifi off if i wanna check my yahoo mail. its kindof annoying but not a dealbreaker by any means.
Make sure Wi-Fi is turned off
Press the E-Mail icon (the default Android E-Mail application)
Enter your Yahoo E-Mail address and password
Press “Manual Setup” in the lower left hand corner
For “Incoming Server Settings” set the IMAP server to “imap.mail.yahoo.com” and the Port to 143
(use tls)