diff --git a/Lib/mailbox.py b/Lib/mailbox.py
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -692,6 +692,13 @@
         self._file = open(self._path, 'rb+')
         self._toc = new_toc
         self._pending = False
+        # Set modification time to be after access time so that MMDF and MBOX
+        # mail readers detect new mail
+        try:
+            currtime = time.time()
+            os.utime(self._path, (currtime-3, currtime))
+        except:
+            pass
         if self._locked:
             _lock_file(self._file, dotlock=False)