[nznog] Xtra temporarily deferred MX

A client is complaining that a user at Xtra is not receiving email they sent. I sent a test email and viewed the MX logs and found the following: mx1.tnz.mail.yahoo.com[124.108.96.67] refused to talk to me: 421 Message from (DELETED) temporarily deferred - 4.16.50. Please refer to http://help.yahoo.com/help/us/mail/defer/defer-06.html This is also appearing on other mail servers with other senders. Is this something Xtra is doing across the board or is there another explanation? The web link showing has no information corresponding to the status result.