diff -r aca1a271c4fc Lib/mailbox.py
--- a/Lib/mailbox.py	Fri Jun 08 13:24:12 2012 -0700
+++ b/Lib/mailbox.py	Sat Jun 09 22:23:07 2012 +0300
@@ -617,7 +617,9 @@
         """Write any pending changes to disk."""
         if not self._pending:
             return
-
+        if self._file.closed:
+            self._pending = False
+            return
         # In order to be writing anything out at all, self._toc must
         # already have been generated (and presumably has been modified
         # by adding or deleting an item).
@@ -1816,6 +1818,10 @@
         else:
             self._pos = pos
 
+    def __del__(self):
+        if hasattr(self,'_file'):
+            self.close()
+
     def read(self, size=None):
         """Read bytes."""
         return self._read(size, self._file.read)
diff -r aca1a271c4fc Lib/test/test_mailbox.py
--- a/Lib/test/test_mailbox.py	Fri Jun 08 13:24:12 2012 -0700
+++ b/Lib/test/test_mailbox.py	Sat Jun 09 22:23:07 2012 +0300
@@ -133,6 +133,7 @@
         msg = self._box.get(key1)
         self.assertEqual(msg['from'], 'foo')
         self.assertEqual(msg.fp.read(), '1')
+        msg.fp.close()
 
     def test_getitem(self):
         # Retrieve message using __getitem__()
@@ -165,10 +166,14 @@
         # Get file representations of messages
         key0 = self._box.add(self._template % 0)
         key1 = self._box.add(_sample_message)
-        self.assertEqual(self._box.get_file(key0).read().replace(os.linesep, '\n'),
+        msg0 = self._box.get_file(key0)
+        self.assertEqual(msg0.read().replace(os.linesep, '\n'), 
                          self._template % 0)
-        self.assertEqual(self._box.get_file(key1).read().replace(os.linesep, '\n'),
+        msg1 = self._box.get_file(key1)
+        self.assertEqual(msg1.read().replace(os.linesep, '\n'), 
                          _sample_message)
+        msg0.close()
+        msg1.close()
 
     def test_get_file_can_be_closed_twice(self):
         # Issue 11700
@@ -403,6 +408,7 @@
         self._box.add(contents[0])
         self._box.add(contents[1])
         self._box.add(contents[2])
+        oldbox = self._box
         method()
         if should_call_close:
             self._box.close()
@@ -411,6 +417,7 @@
         self.assertEqual(len(keys), 3)
         for key in keys:
             self.assertIn(self._box.get_string(key), contents)
+        oldbox.close()
 
     def test_dump_message(self):
         # Write message representations to disk
@@ -1879,7 +1886,9 @@
         self.createMessage("cur")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         #self.assertTrue(len(self.mbox.boxes) == 1)
-        self.assertIsNot(self.mbox.next(), None)
+        msg = self.mbox.next()
+        self.assertIsNot(msg, None)
+        msg.fp.close()
         self.assertIs(self.mbox.next(), None)
         self.assertIs(self.mbox.next(), None)
 
@@ -1887,7 +1896,9 @@
         self.createMessage("new")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         #self.assertTrue(len(self.mbox.boxes) == 1)
-        self.assertIsNot(self.mbox.next(), None)
+        msg = self.mbox.next()
+        self.assertIsNot(msg, None)
+        msg.fp.close()
         self.assertIs(self.mbox.next(), None)
         self.assertIs(self.mbox.next(), None)
 
@@ -1896,8 +1907,12 @@
         self.createMessage("new")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         #self.assertTrue(len(self.mbox.boxes) == 2)
-        self.assertIsNot(self.mbox.next(), None)
-        self.assertIsNot(self.mbox.next(), None)
+        msg = self.mbox.next()
+        self.assertIsNot(msg, None)
+        msg.fp.close()
+        msg = self.mbox.next()
+        self.assertIsNot(msg, None)
+        msg.fp.close()
         self.assertIs(self.mbox.next(), None)
         self.assertIs(self.mbox.next(), None)
 
@@ -1906,11 +1921,13 @@
         import email.parser
         fname = self.createMessage("cur", True)
         n = 0
-        for msg in mailbox.PortableUnixMailbox(open(fname),
+        fid = open(fname)
+        for msg in mailbox.PortableUnixMailbox(fid,
                                                email.parser.Parser().parse):
             n += 1
             self.assertEqual(msg["subject"], "Simple Test")
             self.assertEqual(len(str(msg)), len(FROM_)+len(DUMMY_MESSAGE))
+        fid.close()    
         self.assertEqual(n, 1)
 
 ## End: classes from the original module (for backward compatibility).
diff -r aca1a271c4fc Lib/test/test_old_mailbox.py
--- a/Lib/test/test_old_mailbox.py	Fri Jun 08 13:24:12 2012 -0700
+++ b/Lib/test/test_old_mailbox.py	Sat Jun 09 22:23:07 2012 +0300
@@ -73,7 +73,9 @@
         self.createMessage("cur")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         self.assertTrue(len(self.mbox) == 1)
-        self.assertTrue(self.mbox.next() is not None)
+        msg = self.mbox.next()
+        self.assertTrue(msg is not None)
+        msg.fp.close()
         self.assertTrue(self.mbox.next() is None)
         self.assertTrue(self.mbox.next() is None)
 
@@ -81,7 +83,9 @@
         self.createMessage("new")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         self.assertTrue(len(self.mbox) == 1)
-        self.assertTrue(self.mbox.next() is not None)
+        msg = self.mbox.next()
+        self.assertTrue(msg is not None)
+        msg.fp.close()
         self.assertTrue(self.mbox.next() is None)
         self.assertTrue(self.mbox.next() is None)
 
@@ -90,8 +94,12 @@
         self.createMessage("new")
         self.mbox = mailbox.Maildir(test_support.TESTFN)
         self.assertTrue(len(self.mbox) == 2)
-        self.assertTrue(self.mbox.next() is not None)
-        self.assertTrue(self.mbox.next() is not None)
+        msg = self.mbox.next()
+        self.assertTrue(msg is not None)
+        msg.fp.close()
+        msg = self.mbox.next()
+        self.assertTrue(msg is not None)
+        msg.fp.close()
         self.assertTrue(self.mbox.next() is None)
         self.assertTrue(self.mbox.next() is None)