If you want to send text-only messages you can do this with two steps.
First
Change subscription/email.php to this:
<?php
// TODO: Explain how to customize.
?>
<?php echo $message; ?>
Second
Edit function mail in plugin.php and set all IsHTML
to false
. This will change the header in the mail. Please note that even if you try to send an html E-Mail it will have the wrong header.
Stefano: As your plugin is able to distinct between HTML and Text-Only Newsletters it would be great to be able to choose in which content-type the subcription messages should be.