Yahoo mail servers have been consistently delaying or rejecting e-mails for over a year. You can read about it here:
Or just Google for “4.16.50”.
The short of it is that even with a low volume personal mail server, with the correct spf records, without running an open relay, without being blacklisted by ANY blacklist site/service, Yahoo still won’t deliver e-mail from you. And they aren’t responsive about addressing the problem.
So please, if you use Yahoo mail, switch to GMail. You’ll like it more, and more importantly, you’ll actually get e-mail people are trying to send you.