@wormbrain If an email address is attached to your account for billing etc Optus won't just close them down. So if you haven't do so already setup up a new email address with your preferred provider and register the new email address with your Optus account.
Once you have done that Please message Optus Support 24/7 from the My Optus app. If you do not have access to the My Optus app you can use social media to contact Optus Support, please send a PM to Optus on Facebook or a DM on Twitterand include in the message you do not have access to the My Optus app.
I am not an Optus employee. If a reply helps please give a Kudos and when an issue is solved please mark as solved.