Hi Shelly,
This is the message they get.
Delivery to the following recipient failed permanently:
Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain bigpond.net.au by extmail.bpbb.bigpond.com. [61.9.189.122].
The error that the other server returned was:
550 5.1.1 <@bigpond.net.au> recipient rejected. IB603a