if (!empty($email->getAdditionalMailHeader())) {
foreach ($email->getAdditionalMailHeader() as $name => $values) {
// Set the "Message-ID" header for PHP-Mailer directly
- if ($name == 'Message-Id') {
+ if (strtolower($name) === 'message-id') {
// implode all values to one entry, because there's only one value possible
$mailer->MessageID = trim(implode("", $values));
} else {