Just added my wife's yahoo account to my phone just to see and it works fine. It did have a little trouble verifying. Said the server was not available. After trying again it went through. That is with the default setting through automatic config. The HTC is in the server field. :headscratch:
EDIT: Gotta be something with the specific yahoo account. I just tried putting my non working account on my wife's EVO 3D and it does not work on her phone either. The issue seems to follow the account and not the phone.