Have you tried going to this link.
https://my.bigpond.com/mybigpond/myaccount/consumeremail/default.do
If it doesn't do anything you might need to call up Telstra to keep the mailbox
https://my.bigpond.com/mybigpond/myaccount/consumeremail/default.do
If it doesn't do anything you might need to call up Telstra to keep the mailbox