Bug #178820 --> return-receipt-to not recognized with IMAP mailbox

Porting trunk fix.

mail/base/content/mailWindowOverlay.js

@@ -2342,8 +2342,8 @@ function HandleMDNResponse(aUrl)
   if ((msgFlags & MSG_FLAG_IMAP_DELETED) || (msgFlags & MSG_FLAG_MDN_REPORT_SENT))
     return;
 
-  var DNTHeader = mimeHdr.extractHeader("Disposition-Notification-To", false);
-  if (!DNTHeader)
+  var oldDNTHeader = mimeHdr.extractHeader("Return-Receipt-To", false);
+  if (!DNTHeader && !oldDNTHeader)
     return;
  
   // Everything looks good so far, let's generate the MDN response.