My last response form HTC. Sounds like they are not going to pursue this issue further:
Thank you for your response.
I apologize that the settings I have provided did not resolve your issue. Unfortunately, I can only suggest using the Yahoo Mail application. Do you have any other questions or concerns, while you wait on a response from Yahoo? If not, I will now close your ticket with HC Technical Support. If you receive a response from Yahoo, please feel free to reply or start a new ticket with HTC Technical Support. I do apologize once again for any inconvenience you may have experienced.