If you are receiving the emails on your Gmail app as well then it's possible it's removing the emails from the Optus mail server once they have downloaded. I am not familiar with that email program however there should be an option in the advanced mail settings that allows you to retain the emails on the server.
Edit: I just found the following information that may help.
Log into Gmail with a browser. Go to Mail Settings > Forwarding and POP/IMAP tab. Select Leave a copy on server there.