cert had an invalid/incorrect hostname
fyi,
https://kb.mozillazine.org/Files_and_folders_in_the_profile_-_Thunderbird
...
cert_override.txt
This is an optional file used to store a security
exception. It appears to store the host name , thus preventing you from
creating a security exception for a rotating SMTP server.
...
for ref,
Firefox: How to audit & reset the list of trusted servers/CAs
https://access.redhat.com/solutions/1549043
