Look at the anti virus on your new device. Many of them "hack" secure connections to scan the content before it gets to the application. The result of this is they use dodgy certificates to decrypt mail and Thunderbird does not see them as valid as they are not issued in the chain of trust. They are self signed by the anti virus company. I don't trust any of them.

Much appreciated Matt, but I'm sure what to look for in the Virus and Threat Protection menu options. I'm using Windows Security (that came with Windows 11).

I can not help you with windows 11. My new computer (3 months now) is not compatible apparently.

No worries. Thanks anyway.

I have the same problem with win11. enclosed is a screenshot of the message I received

Last I heard of that (some time ago) it was done by AVAST. Do you have an anti virus that purports to scan secure connections?

norton 360 but it hasn't generated a message before that or windows defender