Mails Incorrectly Being Sent as Plain Text

Drupal does not support HTML email by default, and has never supported HTML email by default - to send HTML email you need to use a contributed module that will do that for you.

The MailSystem module allows you to choose which contributed module(s) will be used for the email formatting task and for the email sending task, but MailSystem does NOT format by itself and does NOT send mail by itself.

Specifically, the "Default PHP Mailer" performs NO formatting and sends mail using PHP as configured on your server. This will not allow you to send HTML email.

Apparently you are using symfony_mailer_lite, since you opened an identical issue here: 💬 Module Isn't Working or Emails Sent as Plain Text Active
Any problems you have using that module to send HTML email should be addressed in the issue queue for that module.

Drupal is following an old standard that breaks lines of text after 78 characters.

This is not an OLD standard, it is THE standard. Using the standard ensures the maximum possible compatibility with email servers and email readers.

Closing this as a duplicate of your other issue.

Thanks for the info, and I understand now. It would be nice if this was part of your README file or project description, so it's more clear what this module does and doesn't do. In this case, it would have saved me various stages of testing and ultimately feeling like I had to post this support request. Anyway, thanks again for the help and I will proceed via the original issue.