I have both mail system and mime mails modules installed on Drupal 8 to do emails from website to users.
I have a custom forward module that contains a custom form that users should use to forward an article to an email address they provide, I am using the extra modules to be able to style and use HTML module.
I got the settings to mail system as follows, I am using the mime mail module as the Formatter to enable HTML emails and the default PHP mailer as the sender.
Note: I tried first to use mime mail as the Formatter AND Sender but this actually did not work for some reason, it couldn't send emails.
This way of using both modules enabled me to use HTML emails and I thought it was working until I found out that it is not working for all email services, for example, it does not work for Gmail (Which is a pretty big deal) Due to this error I found in Drupal's recent log message.
Error sending email (from "MYWEBSITE" <MYEMAIL> to MYTEST@gmail.com with reply-to not set).
It is actually working on my company's email hosted from GODaddy.