(Using 3.5.0) I had to do an HTML mailout yestuday, and had to hack a line to get it to go...
In includes/class_mail.php:
Code:
$headers .= 'Content-Type: text/plain' . iif($encoding, "; charset=\"$encoding\"") . $delimiter;
Change the obvious to:
Code:
$headers .= 'Content-Type: text/html' . iif($encoding, "; charset=\"$encoding\"") . $delimiter;
Something more elegant may come around, but that was my quick fix.