Skip grouping DSNs

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

@@ -1337,6 +1337,12 @@ public class MessageHelper {
 
         boolean subject_threading = prefs.getBoolean("subject_threading", false);
         if (subject_threading) {
+            boolean dsn = false;
+            try {
+                dsn = imessage.isMimeType("multipart/report");
+            } catch (Throwable ignored) {
+            }
+            if (!dsn) {
                 String sender = getSortKey(getFrom());
                 String subject = getSubject();
                 long since = new Date().getTime() - MAX_SUBJECT_AGE * 3600 * 1000L;
@@ -1349,6 +1355,7 @@ public class MessageHelper {
                         }
                 }
             }
+        }
 
         return thread;
     }