diff --git a/lib/xenu_nntp/mbox.py b/lib/xenu_nntp/mbox.py
index ac81c83..0c1372f 100644
--- a/lib/xenu_nntp/mbox.py
+++ b/lib/xenu_nntp/mbox.py
@@ -44,15 +44,20 @@ class MBoxReaderBuffer():
 
     def is_start(self):
         if self.lines[0] is None or self.lines[1] is None:
-            return
+            return False
             
-        if self.is_from_line(0) and self.is_header_line(1):
-            pass
+        if self.is_from_line(0) \
+         and self.is_header_line(1) \
+         and self.lines[2] is None \
+         and self.lines[3] is None:
+            return True
         elif self.is_empty_line(0) \
          and self.is_empty_line(1) \
          and self.is_from_line(2) \
          and self.is_header_line(3):
-            return 3
+            return True
+        else:
+            return False
 
 class MBoxReader():
     __slots__ = 'path', 'fh', 'line', 'buf', 'message',