diff --git a/src/Swift_Transport_PostmarkTransport.php b/src/Swift_Transport_PostmarkTransport.php
index 0113c72..21d229f 100644
--- a/src/Swift_Transport_PostmarkTransport.php
+++ b/src/Swift_Transport_PostmarkTransport.php
@@ -158,6 +158,11 @@ public function send(\Swift_Mime_Message $message, &$failedRecipients = null)
             $data['HtmlBody'] = $html->getBody();
         }
 
+        $tag = $message->getId();
+        if (strlen($tag)) {
+            $data['Tag'] = $tag;
+        }
+
         if ($message->getChildren()) {
             $data['Attachments'] = array();