We're having the exact same problem here since yesterday : using Thunderbird 78.5, and ESET NOD 32 version 14.0.22.0, we can't get any message from our external MAPI accounts (at 1and1/Ionos in France). The sent messages can't be copied in the Sent emails folder either.
Note this is hapening with Windows 10 Pro 64 bits only : the same Thunderbird with the same ESET NOD 32 work fine under Windows 7 Pro 64 bits.
I had to disable temporarly the TLS/SSL certificate checking as a work around, expecting a fix from ESET or Mozilla. The suggestion about switching the checking ON and OFF again, or to use the interactive mode didn't work for me (or only for a few minutes, but I'm not sure I did it right...).
PS : kindly note your captcha bellow doesn't seem to be compatible with Chrome... ;)
