sometimes a reboot will do wonders. have you tried just turning your phone off and then on again? That is usually the first thing I do when i have a problem and more times than not it fixes the problem.
I was able to use yahoo mail via imap for about 2 weeks, then it just stopped no matter what I did. I think they block your account once they cast some kind of a "net" on those users who circumvent their paid service.
My account stopped syncing, saying login failed. Reboot doesn't help, tries to delete and re-add account, unable to validate my info. Confirmed username and password are correct, everything. Tried the K-9 app, says only tmo users can get free yahoo email.
Hey guys, just wanted to let you know that this happened to me also initially a few days ago and then all of a sudden, yesterday it started working again. I didn't do anything differently. Also, by the way, I'm using the "free" account.