Please post a screenshot of the error message. https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Images 1 and 2 come when I launch Thunderbird. Images 3 and 4 appear when I try to send a an email.

Thanks for looking at this!

In the Security Exception window please press the 'Get Certificate' button, and then the 'View' button. The latter will open a new window with the certificate details. Please post a screenshot for that as well.

MMErrorImage5 and MMErrorImage6 show what happened when I press the "Get Certificate" button. it is the same for the POP and SMTP certificates: the "View" button is greyed out in both cases and cannot be pressed so I cannot send you a screenshot of what happens then. This is why I have been concerned all along, I can't even see an outdated certificate?

Thank you for thinking about this.

What is your anti-virus/security software?

I'm using Norton 360

Norton security screenshot of program info from setting attached

Try to start Windows 10 in safe mode with networking enabled.

Still in Windows safe mode, start Thunderbird in safe mode. Does the problem go away?

Windows safe mode disables anti-virus software. This is a troubleshooting step, not a solution.

Hi Christ1,

I'm running Windows 11: I think in order to start in safe mode I am going to have to get in touch with the people who set the computer up and installed it to find out where the Bitlocker key is held. I am not comfortable doing this sort of thing without someone holding my hand: I'll try to get a support session from them this coming week, today is full up with other commitments.

Thanks for thinking about this and having suggestions what to do.

Cheers, Margaret

I think you probably need to lower your security in accordance with the requirements of Norton. They scan using untrusted self signed certificates. I suggest you ask Norton how to add them to the Thunderbird certificate store. Or just don't scan email. I don't.

You might want to exclude Thunderbird from the Norton Software updater if it is included in your package as it has caused essentially forced update to Thunderbird even after the development team cease a rollout because of issues found with the release. Thunderbird will manage that process much more gently that the Norton brute force method.

Dear Christ1 and Matt, I haven't been able to get help to start Windows in safe mode (can't work it out myself), but I tried running Thunderbird again today and this time I actually got a certificate to view, as attached: confirming that it is Norton security which is the 'unknown' certificate issuer. Is it secure to just accept that Norton is scanning emails and set this as a permanent exception? Sorry it has taken me so long to get back onto this, I've been using webmail but it is not sustainable long term, need to get back to Thunderbird! I'm just not confident manipulating Norton settings so at the moment that doesn't seem like an option for me. Thanks very much for thinking about this for me, Margaret

As already suspected, Norton is issuing a cert on the fly, which is rejected by Thunderbird due to an unknown issuer. The fix would be to create an exception in Norton not to scan the Thunderbird profile folder, so that the anti-virus real-time scanner will not scan it. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-to-find-your-profile

Is it secure to just accept that Norton is scanning emails and set this as a permanent exception?

It is secure to to create an exception in Norton not to scan the Thunderbird profile folder. Alternatively, as already suggested by Matt, you can add the Norton cert to the Thunderbird certificate store.