My guess is that it is /include/class.mailer.php circa lines: 434-447 (specifically 442): // If the message is not explicitly declared to be a text message, // then assume that it needs html processing to create a valid text // body $isHtml = true; if (!(isset($options) && $options)) { // Embed the data-mid in such a way that it should be included // in a response if ($options || $mid_token) { $message = sprintf('<div style="display" class="mid-%s">%s</div>%s', $mid_token, $options, $message); }