The errors you got are exactly caused by the problem I explained you above. Yahoo and other servers are not accepting emails from your server because it uses a dynamic IP address.
This is a common problem and the reson why mailservers are normally not run on a dsl line. I listed the two solutions for your problem in my first post.
Take a look here on how to configure your email server to relay emails trough a external mailserver:
http://www.howtoforge.com/postfix_re...her_mailserver