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Same issue here. Cannot send any mails as long as Cardbook Extension is active.
I remember that this problem occured some years ago, also combined with the release of a new TB version. So I think it cannot be any new or unknown reason.

Это отзыв для предыдущей версии этого дополнения (95.3). 

after the available 95.5, a new version is instance of approbation which fix most the known bugs (95.6 here :https://addons.thunderbird.net/fr/developers/addon/cardbook/versions/2491083)

if you have remaining issues, please open a ticket here : https://gitlab.com/CardBook/CardBook/-/issues