That isn't a Thunderbird certificate, that is the email service provider's certificate.

Presumably other customers have reported this to them by now, but I suggest trying their chat support to get an updated status: https://www.dodo.com/contact-us

Hi . Thanks for the reply. I have been in contact with my provider (dodo - unfortunate name but have been reliable for the past 20 years) and they are unable to help me. I can access all my emails by logging into the web browser. However, on my desktop computer which is running Mozilla Thunderbird, the emails are not coming through and I am still getting the message I posted previously. Just wondering why you think it is the email providers certificate and not Thunderbird's when the error message has the Mozilla icon.

I've never had this problem but I've been digging around a bit. Certificate failures don't appear to be uncommon and there are pages out there suggesting reasons and the actions to resolve them. For example: https://windowsreport.com/email-certificate-error/ However, one possibility is that the pop server name has been changed and the old server's security has expired. I checked the Dodo Pop3 settings and they seem to be different to the one you're using (use the Pop3) tab: https://www.getmailbird.com/setup/access-dodo-com-au-via-imap-smtp

While my posted solution is being checked over by a moderator (again! sigh...), I'd check out the pop3 server settings. A little research suggests that the server name has changed and this may be causing the error.

Hi . Thanks for the reply. I have been in contact with my provider (dodo - unfortunate name but have been reliable for the past 20 years) and they are unable to help me.

Do they have updated connection information to retrieve mail using the POP3 protocol?

Just wondering why you think it is the email providers certificate and not Thunderbird's when the error message has the Mozilla icon.

The error is that Thunderbird can't connect to the pop3 server because the pop3 server's certificate has expired.

However, one possibility is that the pop server name has been changed and the old server's security has expired. I checked the Dodo Pop3 settings and they seem to be different to the one you're using (use the Pop3) tab: https://www.getmailbird.com/setup/access-dodo-com-au-via-imap-smtp

I don't know whether that mailbird site is correct, since I don't know how Telstra and Dodo are related. But hopefully Dodo support can confirm/deny those settings before you make changes. Or maybe they are forcing everyone to use webmail now??