I thought the email failed when sending TO Yahoo, Hotmail and AOL? So it shouldn't block when sending to the the email tester account.
Where is the Yahoo, Hotmail and AOL email account being used in the email when it fails? Just in the body of the email? I'm a bit confused now, so I need a little more information. Do you have a url and FTP information I can use to debug? I'll also want steps to reproduce since I'm not quite sure where the email addresses are used that cause the problem.