I am also having problems with emails not sending to bigpond accounts. I have been asking for delivery and read receipts but now they come back delivered but not everyone is sending back read receipts which makes me think they are not receiving the emails.
Automatic read recipients are a bad idea and most people I know won't allow them. Have you asked any of those Telstra recipients if your mail is in their spam folder as Optus mail servers are often flagged for spam.
Any bounced or rejected emails coming back to you should give you a reason why they were rejected. I believe emails that have not been rejected would either be in their spam folder or they have chosen not to give you a read receipt.