Open Proxies and open relays are a very common source of spam. Yahoo uses a variety of techniques to detect these, and Yahoo does not accept SMTP connections from either open relays or open proxies. Your connection may be rejected because the email server that you use for sending email is probably an "open relay" or "open proxy." These types of email servers can easily be tricked into sending large amounts of unsolicited email.
Many email services refuse mail from such servers, until the server has been fixed to stop relaying spam to innocent users.
To correct this problem, you should contact the system administrator of your mail server (usually "firstname.lastname@example.org") and forward them a copy of the bounced email you received. Otherwise, you should call technical support and alert them of the problem.
You can check to see if your mail server is listed on several of the public open relay DNSBLs. Please note that if your server is not listed at this link, it may still be an open proxy or open relay. Abuse.net provides a web-based open relay test. Instructions to close open relays are readily available on the Internet.