Ya'...thats a tough one! There is no resolution from the service carriers point of view. That is a straight Motorola question/problem.
I have the problem whenever I am trying to help a client through the Terms & Conditions for the Boost Premium Plan from there handset, if they have an underscore I have to change the e-mail address to a default of boost@boost.com!
Personally..I am not a fan of the Motorola as a text platform, heavy handed and slow, but what do you do?
The pro-active resolution for you is to change your yahoo address, inconvienient, but it is a resolution...sorry.
Jess