We are having the exact same issue, and were also previously having this issue back in Jan (see Link Below)
https://crowdsupport.telstra.com.au/t5/Telstra-and-or-BigPond-Email/Bigpond-mail-is-rejecting-emails-from-my-whitelist-sender/td-p/639902
previously the issue was only related to Bigpond.net.au but now is with .net & .com
Our mail is hosted with Office 365 (this seemed to be a common connection last time the issue was around)
See below rejection email
This message is a warning that an email you are trying to send has not yet been delivered. You do not have to do anything yet as the email is still queued for delivery.
An email you addressed to email address :
-- ***********@bigpond.net.au
has not yet been delivered. The problem appears to be :
-- Recipient server unavailable or busy
Additional information follows :
-- Connection refused. IB113
This condition occurred after 6 attempt(s) to deliver over a period of 1 hour(s).
You will receive a rejection notice after a maximum of 4 days if the email cannot be delivered. If you sent the email to multiple recipients you will receive one of these messages for each one which is still undelivered, otherwise they have been sent.
** address recipient has been changed