Hi. This is the qmail-send program at yahoo.com. I'm afraid I wasn't able to deliver your message to the following addresses. This is a permanent error; I've given up. Sorry it didn't work out.
This error means that some Spam email has been sent from your IP Address(may be a spammer faked your IP Address), and so your Service Provider has blacklisted your IP Address to avoid further Spam mails. You have to get into the site: http://psbl.surriel.com/listing?ip=68.142.199.171 and remove the spam filter first. Then contact AT&T support for further assistance.
I would also recommend that you change your yahoo mail password from a computer that you know is clean and not infected with any trojan keyloggers.
We had a spammer using an account once to send email even when we changed the password they still got back into it, after awhile we changed the password from another place and the problem stopped.