Thunderbird is advising you that the Security for the site is unverified and needs updating. You can allow an exception so that Thunderbird accesses the site but that is never advisable.

Optusnet has been throwing up problems with TB in the past couple of weeks. A link to the Optus help sheet is here - https://support.mozilla.org/en-US/questions/1486768. I note that the TB settings there posted are different to the ones being used.

Thanks for that. Is there a way to update the Security for the site then? I cant see the answer in the other threads :(

Thanks for that. Is there a way to update the Security for the site then? I cant see the answer in the other threads :(

It's nothing to do with either you or Thunderbird - TB is just reporting the problem that the security certificate hasn't been updated. Personally, I wouldn't allow the connection but...

...there have been numerous problems with connections to Optusnet recently. Not sure if this will help but there's a link to an Optusnet help page at the bottom - https://support.mozilla.org/en-US/questions/1486768. As you'll see, the SMTP server there is different to the one you are trying to access.

There are also problems with Third Party Anti-virus software that are effectively causing connections to hang and fail (which doesn't appear to be your problem).

We checked the SMTP setting with Optus today and they are correct.

Well, according to their own website, they're not and outgoing server name should be the same as for the incoming server (mail.optusnet.com.au):

https://www.optus.com.au/support/answer?id=1434