Found this on page 62 of ROM Thread was very helpful for me. Credit goes to Inkarnit.
Go to manual setup > IMAP account and change to these settings....
Imap server - imap.mail.yahoo.com
Click Next... Wait for it verify
SMTP Server- smtp.mobile.mail.yahoo.com
When I try this, I keep getting an error that I have an incorrect username/password...but I am positive I am entering them in correctly. I'm not really sure what to make of it
Make sure you are not connected to wifi and that you are keying the info correctly. On the incoming server scree type username with @yahoo.com. on the outgoing server screen check the box require sign in type you username without @yahoo.com. see if that works.