cannot send email through AT&T


New Email
I am getting this error message when I try to send email through our ISP (ATT/Yahoo). The link does not provide any help.

554 Transaction failed : Cannot send message due to possible abuse; please visit Yahoo! Help | Customer Care - How can we help you? for more information

Before we were getting this, we were getting a bounceback message that our domain name email server was unreachable for too long. Our domain name support said the problem was on Yahoo's end.