This appears to be because some anti spam service has decided the Godaddy server is the source of too much spam.
They are simply looking at the IP address the email is coming from, which is shared by thousands of Godaddy customers.
I have complained to the blocking service, to no avail...
The bounce message look like this:
211.29.133.14 does not like recipient.
Remote host said: 521 5.7.1 <####@optusnet.com.au>: Recipient address rejected: You are not allowed to send mail. Please see http://csi.cloudmark.com/reset-request/ ... 202.161.71 if you feel this is in error.
Giving up on 211.29.133.14.