Thank you for your continued support. Some HTML emails are being displayed as plain text source code rather than rendered properly. Standard HTML emails work fine, but I run a personal e-commerce site using WordPress, and all notification emails sent from that site appear as source code. When viewed on other tablets or iPhones, these emails display correctly with the markup intact. Furthermore, HTML emails from other sites show no issues and display correctly in Thunderbird. 　My operating system is Windows 11 Home, and I am using the latest version of Thunderbird. Any advice you could offer would be greatly appreciated.

Do you use Bitdefender? If not, does Repair Folder from the folder Properties (context menu) help?

Thank you for your advice.

I don't use Bitdefender, and use Ansin-net security by KDDI.
And as you indicate,I repair folder from the folder properties,but the HTML mails have still being displayed as plain text.

Since this only happens with WordPress mail, I can only surmise that there is some non-standard html in those messages that TB is unable to read. Without knowing the message source, does changing View/Message Body As to something other than Original HTML make them readable? Press Alt if the Menu Bar with View is hidden.

Thank you for your return.

Trying to change View/Message Body As to the others, the problem is not solved.
To Display Source of the mail which is not displayed correctly from View,text file is opened with the source code,and if I save it as html file and open with browser,the file is correctly opened except the header as the
attachment. 
So source may be not uncorrect.