diff -r 3a57eafd8401 Lib/mailbox.py
--- a/Lib/mailbox.py	Tue May 24 09:15:14 2016 +0300
+++ b/Lib/mailbox.py	Tue May 24 22:03:25 2016 +0200
@@ -23,9 +23,10 @@
 except ImportError:
     fcntl = None
 
-__all__ = [ 'Mailbox', 'Maildir', 'mbox', 'MH', 'Babyl', 'MMDF',
-            'Message', 'MaildirMessage', 'mboxMessage', 'MHMessage',
-            'BabylMessage', 'MMDFMessage']
+__all__ = ['Mailbox', 'Maildir', 'mbox', 'MH', 'Babyl', 'MMDF',
+           'Message', 'MaildirMessage', 'mboxMessage', 'MHMessage',
+           'BabylMessage', 'MMDFMessage', 'Error', 'NoSuchMailboxError',
+           'NotEmptyError', 'ExternalClashError', 'FormatError']
 
 linesep = os.linesep.encode('ascii')
 
diff -r 3a57eafd8401 Lib/test/test_mailbox.py
--- a/Lib/test/test_mailbox.py	Tue May 24 09:15:14 2016 +0300
+++ b/Lib/test/test_mailbox.py	Tue May 24 22:03:25 2016 +0200
@@ -2268,12 +2268,18 @@
 """)
 
 
+class MiscTestCase(unittest.TestCase):
+    def test__all__(self):
+        blacklist = {"linesep"}
+        support.check__all__(self, mailbox, blacklist=blacklist)
+
+
 def test_main():
     tests = (TestMailboxSuperclass, TestMaildir, TestMbox, TestMMDF, TestMH,
              TestBabyl, TestMessage, TestMaildirMessage, TestMboxMessage,
              TestMHMessage, TestBabylMessage, TestMMDFMessage,
              TestMessageConversion, TestProxyFile, TestPartialFile,
-             MaildirTestCase, TestFakeMailBox)
+             MaildirTestCase, TestFakeMailBox, MiscTestCase)
     support.run_unittest(*tests)
     support.reap_children()