Internode also moved their email service over to The Messaging Company recently: https://www.internode.on.net/nodesupport/faq/internode_email_messaging_company Could be why a 550 code was thrown up as well, as they've stopped providing that service.

Could also be that your email got flagged as spam and was rejected on that basis as well. I wouldn't automatically jump to ISPs, though given that Internode is an ISP based in Australia...

Not necessarily. See the following for more information on error code 550: https://www.scalahosting.com/kb/smtp-error-550/#:~:text=In%20simplest%20terms%2C%20the%20550,code%20message%20has%20different%20variations