I have the same problem also it is not possible to put a enter or something like that.
Also emails are being sent out with the From address as “wordpress” and the email as firstname.lastname@example.org – would be good to be able to adjust this…
Please use an SMTP plugin to fix this.
For HTML email, you have to edit this line: email-header. Replace text/plain to text/html. And then you can add html to your email message.