This should finally fix #6896. Let's watch the buildbots.
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 2abb354..548850a 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -684,6 +684,9 @@
self.assertEqual(self._box._lookup(key0), os.path.join('new', key0))
os.remove(os.path.join(self._path, 'new', key0))
self.assertEqual(self._box._toc, {key0: os.path.join('new', key0)})
+ # Be sure that the TOC is read back from disk (see issue #6896
+ # about bad mtime behaviour on some systems).
+ self._box.flush()
self.assertRaises(KeyError, lambda: self._box._lookup(key0))
self.assertEqual(self._box._toc, {})