Since the 30.11.2020 we are monitoring reports of missing encrypted ports when using e-mail client Thunderbird in the versions 78.4.x and 78.5 and ESET Internet Security at the same time.
If this is your case as well, we would recommend to contact ESET customer support or try to solve the problem with following solutions:
It was recommended by ESET via blog to turn off the SSL/TLS encryption and back on and restart the device.
In the dire need of working with the e-mails you can always use our webmails client RoundCube at mail.active24.eu.
Unavailability of encryption are not present on older versions, so other solution is to downgrade Thunderbird to an older version (excluding the above mentioned) or use alternative e-mail client. Guide for setup are here.
ESET support recommends other solutions which you can find at forum.eset.com, for example:
https://forum.eset.com/topic/26355-thunderbird-7843-email-fetching-problem/?fbclid=IwAR15TuREZJlXHDZiUsfFIIns3aN5gvjNqinnz–LVizZj75zbwU12VK7VAA
https://forum.eset.com/topic/26517-problems-with-thunderbird-and-imaps/