When connecting to an Aussie Broadband hosted mailbox via SMTP from external to the Aussie Broadband network, you may receive a 5.7.1 (message refused) error with the following message:

Data command rejected: SASL Policy rejection - Contact Aussie Broadband on 1300 880 905

This is because Aussie Broadband blocks external (even authenticated) access to send as its hosted mailboxes via SMTP through its own servers. This block must be lifted by contacting Aussie Broadband on the number listed. It took a couple of calls before I was able to get someone who was able to help me, but after they flicked the switch, messages flowed within about 5 minutes.