Add a deprecation notice to mailbox' old classes' doc.
diff --git a/Doc/library/mailbox.rst b/Doc/library/mailbox.rst
index 38765ed..79fe72f 100644
--- a/Doc/library/mailbox.rst
+++ b/Doc/library/mailbox.rst
@@ -1507,11 +1507,13 @@
Deprecated classes and methods
------------------------------
+.. deprecated:: 2.6
+
Older versions of the :mod:`mailbox` module do not support modification of
mailboxes, such as adding or removing message, and do not provide classes to
represent format-specific message properties. For backward compatibility, the
older mailbox classes are still available, but the newer classes should be used
-in preference to them.
+in preference to them. The old classes will be removed in Python 3.0.
Older mailbox objects support only iteration and provide a single public method: