You use telstra and expect a sane and reasonable process. I stopped using their email decades ago (perhaps only one) just as I stopped using their hopelessly incompetent support line. Neither actually worked as advertised and to make matters worse the support folk were in total ignorance of anything that was not in the script they read from and having matter elevated to level two support where folk that actually knew what email was worked generally took a complaint to the ombudsman. Zero real support from the telstra support line. I used to just end up fighting with them as the fumbled and transferred me yet again to someone else in the hope of loosing the call and generally made me aware that I was not in control and they had not idea who actually was and cared even less. I think I recall a recent change to their service where they changed all of their server settings for old bigpond accounts to use telstra branded servers. So let's start with you advising what settings you have in account settings for incoming and outgoing server (SMTP) and we can compare them to those published by telstra here https://www.telstra.com.au/support/email/imap-pop-smtp-mail-server-settings