diff --git a/src/PHPMailer/PHPMailer.php b/src/PHPMailer/PHPMailer.php
index ed0761d..9544bf9 100644
--- a/src/PHPMailer/PHPMailer.php
+++ b/src/PHPMailer/PHPMailer.php
@@ -1113,8 +1113,13 @@ class PHPMailer {
     else {
       $result .= sprintf("Message-ID: <%s@%s>%s", $uniq_id, $this->ServerHostname(), $this->LE);
     }
-    $result .= $this->HeaderLine('X-Priority', $this->Priority);
-    $result .= $this->HeaderLine('X-Mailer', 'PHPMailer ' . $this->Version . ' (phpmailer.sourceforge.net)');
+
+    // The header X-Mailer and X-priority will be enabled if the SMTPDebug is 
+    // activated.
+    if ($this->SMTPDebug) {
+      $result .= $this->HeaderLine('X-Priority', $this->Priority);
+      $result .= $this->HeaderLine('X-Mailer', 'PHPMailer ' . $this->Version . ' (phpmailer.sourceforge.net)');
+    }
 
     if ($this->ConfirmReadingTo != '') {
       $result .= $this->HeaderLine('Disposition-Notification-To', '<' . trim($this->ConfirmReadingTo) . '>');