Just to confirm, what email client are you using with your OptusNet email address? Are you also able to confirm the device it's being used on? It would be worth taking a look at the advanced mail server settings on both your computer and phone and untick "remove from server after a certain period of time" as this would indicate that you may have accidentally set a a timer.
We would also recommend logging into your email via webmail → https://webmail.optusnet.com.au/ to have a look if you are able to access emails older than 31/07/2020.
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