AT&T Error Code Received When Trying to Verify Email Address for Forwarded Emails

Just signed up for Yahoo Mail Plus and keep getting AT&T Error Code while trying to verify the email address to which my AT&T emails are to be forwarded. Have spoken with 4 different AT&T support members and none of them could figure out what to do. The last representative instructed me to call Yahoo. The Yahoo representative directed me back to AT&T.