We had the same issue, found the fix in here:
https://www.drupal.org/project/drupal/issues/3270647 🐛 PhpMail : broken mail headers in PHP 8.0+ because of LF characters Fixed
Sending webform emails without any add-on modules (no SMTP email or anything). On dev they get sent and delivered as HTML just fine out of the box. On a new VPS the emails show up with all the html tags in plain text. Obviously it is something about the server but I cannot figure out what. How do I find out what application is being used to send the email? Is it PHPsendmail? I believe the testing I have done eliminates the email client from being the problem.
Any ideas how to troubleshoot this would be greatly appreciated.
Closed: duplicate
6.1
Miscellaneous
Not all content is available!
It's likely this issue predates Contrib.social: some issue and comment data are missing.
We had the same issue, found the fix in here:
https://www.drupal.org/project/drupal/issues/3270647
🐛
PhpMail : broken mail headers in PHP 8.0+ because of LF characters
Fixed