t_reply() must be used insted of sl_reply() if the transaction

already exists.

etc/ser-oob.cfg

@@ -529,7 +529,14 @@ route[FORWARD]
 	# send it out now; use stateful forwarding as it works reliably
 	# even for UDP2TCP
 	if (!t_relay()) {
-		sl_reply_error();
+		if (isflagset(FLAG_FAILUREROUTE)) {
+			# This should be replaced with
+			# t_reply_error() similar to sl_reply_error()
+			# in order to return the proper failure code.
+			t_reply("500", "Request cannot be forwarded");
+		} else {
+			sl_reply_error();
+		}
 	}
 	drop;
 }