Typically when SmarterMail fails to deliver a message to Yahoo, or Yahoo bounces the message, there are error details contained in the errors that are indicative of the reason for failure. Do you have this information handy? Also, the following article may help in tracking this down and correcting the issues.
If you continue to experience errors sending mail to Yahoo, it may be due to your server address being listed on blacklists, in which case you'll need to rectify the issues that got you added to the list in the first place, and then request delisting with the individual blacklist maintainers.
If none of the above resolves this issue, and/or you do not have error text/bounce messages available, you may need to enter a support ticket so we can look into this further. I hope this has helped point you in the right direction.