i had the yahoo mail app however i have found it to use a lot of battery power. i dropped it and went back to the stock email app.
i tried the settings up in this thread. the only changes i did were to remove "htc". id make sure there are not spaces in front of the "imap" or "smtp" words. i had left a space before the "imap" (so it was actually "_imap to the app) and that caused things to not work. i re checked.. all is fine...