Debug only

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -1399,8 +1399,9 @@ public class MessageHelper {
         }
 
         // https://docs.microsoft.com/en-us/openspecs/exchange_server_protocols/ms-oxomsg/9e994fbb-b839-495f-84e3-2c8c02c7dd9b
-        String tindex = imessage.getHeader("Thread-Index", null);
+        if (BuildConfig.DEBUG)
             try {
+                String tindex = imessage.getHeader("Thread-Index", null);
                 if (tindex != null) {
                     boolean outlook_thread_id = prefs.getBoolean("outlook_thread_id", false);
                     if (outlook_thread_id) {