Click the view button. Who is the issuer on the certificate. (not that there should be one on Port 25, but Optus are still in the 1990s security wise).. My guess is it is your antivirus program and you need to click the confirm button. But you should not do that unless you trust the issuer.

Thank you Matt for answering my query - tried what you suggested and the issuer was Avast AV so clicked the confirm button. Tried this morning to send email but now comes up with another message (see screenshot) so still cannot send emails. Looking forward to a solution!