Just got off the phone with HTC. Their answer is to download the Yahoo app. They say even if your yahoo account is working currently it more than likely will stop working soon. His exact response was "I am surprised you got an account to work at all. There are known compatibility issues with the mail app and yahoo."
This is a deal breaker for me. I check other exchange and POP3 accounts as well. I want to check them all in one place. Call me stubborn but it is just the way I work. I'm really bummed about this. We have been waiting for this phone to make the move from AT&T and the iphone. Not sure what I am going to do now. I'll keep an eye out here for a week or so. Hopefully someone finds a solution.