Recently, Thunderbird upgraded to version 78.4.0, from version 68. After the update, I couldn't receive any email, from any server whatsoever.
After considrerable reserach, I disabled "ssl/tls protocol filtering" in Eset Internet Security. Now I can receive emails without any issue.
However my doubt remains: have I broken something? What does "ssl/tls protocol filtering" actually do? Is it something useful? Should I turn it back on, and search for the right settings, so I can receive emails but leave the setting on as a whole?