I got this error a lot of times, when i try sending emails from yahoo, when i use the stock app. receiving emails works fine, i see emails...but if i try to reply, the email remains in outbox and "error signing in" keeps appearing in notification bar. the password is correct, i tried also not requesting password when sending, but the error appears.
except other email apps (i use also yahoo app, but i don't like it, and i don't want to sync non stop) and advices to uses to gmail. i have both for different reasons.
i tried also searching for different imap settings, but i could not find a proper one.