EMAIL_HEADER_HTML
Is the name of the field but sometimes it will not find it. I guess it depends on what browser you are using.
The language file is mailer.php.
Find the EMAIL_HEADER_HTML string and add whatever html code you want to it.
So cp>>Languages>Language editor
Choose Specific Language File: Mailer.php
Scroll down till you see EMAIL_HEADER_HTML
Edit the content.
