--- Lib/mailbox.py.1	2007-04-13 13:18:05.000000000 +0100
+++ Lib/mailbox.py	2007-04-13 13:33:15.000000000 +0100
@@ -1388,6 +1388,13 @@
         stop = self._file.tell()
         return (start, stop)
 
+    def _append_message(self, message):
+        """Extend superclass method to write preamble if file is empty."""
+        self._file.seek(0, 2)
+        if self._file.tell() == 0:
+            self._pre_mailbox_hook(self._file)
+        return _singlefileMailbox._append_message(self, message)
+
 
 class Message(email.message.Message):
     """Message with mailbox-format-specific properties."""