Thanks for reaching out; It looks like you have done, a bit of troubleshooting already. We have put together some instructions that may help you reduce the webmail quota warning.
You may also try deleting your spam folder as this can contribute additional storage.
You can Messageus via the My Optus app. You’ll just need to ensure that you select the relevant prompts, Please note, the team won’t be able to respond right away, but they’ll be with you as soon as they can.