Hers what I found and it seems to be working fine for me... the stock email.. delete and start over, chose manual setup, then imap.. delete the samsung from the imap server this is what you should have..
imap server...IMAP.MAIL.YAHOO.COM
security .. SSL
port.... 993
for the out going server
smtp..SMTP.MAIL.YAHOO.COM
security...SSL
port....465
IMPORTANT........UNCHECK THE REQUIRE SIGN IN BOX,
I think that's the main problem, haven't had time to play with it but after disconnecting from wifi for an hour no fail sign in...