Why you may receive a delivery failure message when sending email messages to Yahoo! from a third-party service.

This error message indicates we are seeing a high volume of messages from your IP address that is a characteristic of unsolicited, bulk emails. While this is a temporary SMTP error code, we do not recommend attempting to resend your messages until you examine your subscription practices and lists, and implement changes to ensure that your messages are sent only to users who have requested it. 

If you have deployed significant changes or you have received this error for more than 48 hours, and you are the administrator for the message content or mailing policies, we ask that you review outgoing messages for potentially objectionable content or practices.

Please know that Yahoo! is unable to disclose specific information regarding our filtering practices; however, we do publish recommended guidelines for your review.

If you are not the administrator of the mail server in question, please contact the administrator directly with the error message you are receiving.