diff --git a/src/Swift_Transport_PostmarkTransport.php b/src/Swift_Transport_PostmarkTransport.php
index 0113c72..ff2189f 100644
--- a/src/Swift_Transport_PostmarkTransport.php
+++ b/src/Swift_Transport_PostmarkTransport.php
@@ -143,6 +143,7 @@ public function send(\Swift_Mime_Message $message, &$failedRecipients = null)
         switch ($message->getContentType()) {
             case 'text/html':
             case 'multipart/alternative':
+            case 'multipart/mixed':
                 $data['HtmlBody'] = $message->getBody();
                 break;
             default: