Thanks for the threads and they have given the answers i was after. I know what I have to do now, but I am disappointed. I am sure thousands of Optus customers are not aware that port 110 for incoming POP and 25 for outbound emails are not secure and their emails are being transmitted unencrypted. How could Optus only require port 25 to be used for the outgoing mail server knowing the security risks of the customers' emails being read by third parties is beyond me.
Anyway thank you for your assistance.
I understand that @MAY2, to be honest with you support for Optusnet email has declined in recent years. We find that customers are more inclined to use other email services such as Gmail or Outlook. It's definitely a valid concern though and one that I'll be sure to feedback.