If you have a look at the link provided by BenMcr - there's advice on how to export the Avast certificate so it can be imported into your mail program.
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Sadly, my version of Avast does not seem to have the menu which the URL you give points to.I have found the closest part of the menu system, but there is no facility for exporting a certificate.
The obvious expedient is to tell Thunderbird that the suspect certificate (which is indeed from Avast) is in fact OK:that is, make it an exception.
Is this wise?
FWIW, I had endless problems with Avast a while ago, which I reported on this forum. All of the problems went away after I ditched Avast in favour of Avira.
Ditching Avast was an ordeal - it was insidious and needed a rummage around in the registry to fully release it's grip on the system, regular uninstall routines didn't cut it.
Not a single problem with Avira.
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