diff --git a/mail/base/content/mailWindowOverlay.js b/mail/base/content/mailWindowOverlay.js
index 852cae329bcc..f0e7d955f0c1 100644
--- a/mail/base/content/mailWindowOverlay.js
+++ b/mail/base/content/mailWindowOverlay.js
@@ -2363,6 +2363,7 @@ function HandleMDNResponse(aUrl)
   if ((msgFlags & MSG_FLAG_IMAP_DELETED) || (msgFlags & MSG_FLAG_MDN_REPORT_SENT))
     return;
 
+  var DNTHeader = mimeHdr.extractHeader("Disposition-Notification-To", false);
   var oldDNTHeader = mimeHdr.extractHeader("Return-Receipt-To", false);
   if (!DNTHeader && !oldDNTHeader)
     return;