diff -r a1dea7cde58f Lib/mailbox.py
--- a/Lib/mailbox.py	Thu Apr 07 22:27:44 2011 +0300
+++ b/Lib/mailbox.py	Thu Apr 07 23:11:51 2011 +0200
@@ -20,6 +20,7 @@
 import email.message
 import email.generator
 import io
+import contextlib
 try:
     if sys.platform == 'os2emx':
         # OS/2 EMX fcntl() not adequate
@@ -76,7 +77,8 @@
         if not self._factory:
             return self.get_message(key)
         else:
-            return self._factory(self.get_file(key))
+            with contextlib.closing(self.get_file(key)) as file:
+                return self._factory(file)
 
     def get_message(self, key):
         """Return a Message representation or raise a KeyError."""
diff -r a1dea7cde58f Lib/test/test_mailbox.py
--- a/Lib/test/test_mailbox.py	Thu Apr 07 22:27:44 2011 +0300
+++ b/Lib/test/test_mailbox.py	Thu Apr 07 23:11:51 2011 +0200
@@ -540,8 +540,7 @@
     def _get_lock_path(self):
         # Return the path of the dot lock file. May be overridden.
         return self._path + '.lock'
-
-
+        
 class TestMailboxSuperclass(TestBase):
 
     def test_notimplemented(self):
@@ -1204,6 +1203,37 @@
         self.assertEqual(set(self._box.get_labels()), set(['blah']))
 
 
+class FakeFileLikeObject:
+    
+    def __init__(self):
+        self.closed = False
+
+    def close(self):
+        self.closed = True
+
+
+class FakeMailBox(mailbox.Mailbox):
+    
+    def __init__(self):
+        mailbox.Mailbox.__init__(self, '', lambda file: None)
+        self.files = [FakeFileLikeObject() for i in range(10)]
+    
+    def get_file(self, key):
+         return self.files[key]
+
+
+class TestFakeMailBox(unittest.TestCase):
+    
+    def test_closing_fd(self):
+        box = FakeMailBox()
+        for i in range(10):
+            self.assertFalse(box.files[i].closed)
+        for i in range(10):
+            box[i]
+        for i in range(10):
+            self.assertTrue(box.files[i].closed)
+
+
 class TestMessage(TestBase):
 
     _factory = mailbox.Message      # Overridden by subclasses to reuse tests
@@ -2116,7 +2146,7 @@
              TestBabyl, TestMessage, TestMaildirMessage, TestMboxMessage,
              TestMHMessage, TestBabylMessage, TestMMDFMessage,
              TestMessageConversion, TestProxyFile, TestPartialFile,
-             MaildirTestCase)
+             MaildirTestCase, TestFakeMailBox)
     support.run_unittest(*tests)
     support.reap_children()