At one stage you could manage email accounts via the old Optus account portal however I noticed on the link below that the account management takes you to the new system. I have several email accounts and I have not found anywhere in the new system that manages them.
I would suggest contacting support and getting them to remove the email account if you can't find anywhere to manage them and you can do that via the second link if you wish.
Thanks for your response. I did an online chat today with Optus and was advised they have to delete any secondary email addresses, as you were quite right in that they have changed their systems