- foreach (explode("\n", trim($email->getAdditionalMailHeader())) as $header_line) {
- list($name, $value) = explode(':', $header_line, 2);
- $mailer->addCustomHeader(trim($name), trim($value));
+ foreach ($email->getAdditionalMailHeader() as $name => $values) {
+ // Set the "Message-ID" header for PHP-Mailer directly
+ if (strtolower($name) === 'message-id') {
+ // implode all values to one entry, because there's only one value possible
+ $mailer->MessageID = trim(implode("", $values));
+ } else {
+ $mailer->addCustomHeader(trim($name), trim(implode("\n", $values)));
+ }