misconversion (mojibake) of some mails in Japanese

misconversion (mojibake) does not happen when a mail header is as below. Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: base64 misconversion (mojibake) happens when a mail header is as below. Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit It seems encoding in my Thunderbird does not match 8bit. Several years ago Thunderbird had an option to modify text encoding but I cannot find such an option in the recent versions. It would be really appreciated if anyone help. Thank you.