diff -r 3ae2cd85a908 Doc/library/email.policy.rst
--- a/Doc/library/email.policy.rst	Sun Mar 09 11:18:16 2014 +0100
+++ b/Doc/library/email.policy.rst	Sun Mar 09 23:42:42 2014 +0100
@@ -187,6 +187,16 @@
       :const:`False` (the default), defects will be passed to the
       :meth:`register_defect` method.
 
+
+   .. versionadded:: 3.4
+          The *mangle_from_* parameter.
+
+          .. attribute:: mangle_from\_
+
+            If :const:`True`, lines starting with *"From "* in the body are
+            escaped by putting a ``>`` in front of them.
+            Default: :const:`True`.
+
    The following :class:`Policy` method is intended to be called by code using
    the email library to create policy instances with custom settings:
 
diff -r 3ae2cd85a908 Lib/email/_policybase.py
--- a/Lib/email/_policybase.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/email/_policybase.py	Sun Mar 09 23:42:42 2014 +0100
@@ -149,12 +149,17 @@
                            during serialization.  None or 0 means no line
                            wrapping is done.  Default is 78.
 
+    mangle_from_        -- a flag that, when True escapes From_ lines in the
+                           body of the message by putting a `>' in front of
+                           them. Default: True.
+
     """
 
     raise_on_defect = False
     linesep = '\n'
     cte_type = '8bit'
     max_line_length = 78
+    mangle_from_ = True
 
     def handle_defect(self, obj, defect):
         """Based on policy, either raise defect or call register_defect.
diff -r 3ae2cd85a908 Lib/email/generator.py
--- a/Lib/email/generator.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/email/generator.py	Sun Mar 09 23:42:42 2014 +0100
@@ -36,16 +36,16 @@
     # Public interface
     #
 
-    def __init__(self, outfp, mangle_from_=True, maxheaderlen=None, *,
+    def __init__(self, outfp, mangle_from_=None, maxheaderlen=None, *,
                  policy=None):
         """Create the generator for message flattening.
 
         outfp is the output file-like object for writing the message to.  It
         must have a write() method.
 
-        Optional mangle_from_ is a flag that, when True (the default), escapes
-        From_ lines in the body of the message by putting a `>' in front of
-        them.
+        Optional mangle_from_ is a flag that, when True (the default if policy
+        is not set), escapes From_ lines in the body of the message by putting
+        a `>' in front of them.
 
         Optional maxheaderlen specifies the longest length for a non-continued
         header.  When a header line is longer (in characters, with tabs
@@ -59,6 +59,9 @@
         backward compatibility.
 
         """
+
+        if mangle_from_ is None:
+            mangle_from_ = True if policy is None else policy.mangle_from_
         self._fp = outfp
         self._mangle_from_ = mangle_from_
         self.maxheaderlen = maxheaderlen
@@ -450,7 +453,7 @@
     Like the Generator base class, except that non-text parts are substituted
     with a format string representing the part.
     """
-    def __init__(self, outfp, mangle_from_=True, maxheaderlen=78, fmt=None):
+    def __init__(self, outfp, mangle_from_=None, maxheaderlen=78, fmt=None):
         """Like Generator.__init__() except that an additional optional
         argument is allowed.
 
diff -r 3ae2cd85a908 Lib/email/policy.py
--- a/Lib/email/policy.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/email/policy.py	Sun Mar 09 23:42:42 2014 +0100
@@ -75,6 +75,7 @@
     refold_source = 'long'
     header_factory = HeaderRegistry()
     content_manager = raw_data_manager
+    mangle_from_ = False
 
     def __init__(self, **kw):
         # Ensure that each new instance gets a unique header factory
diff -r 3ae2cd85a908 Lib/test/test_email/test_policy.py
--- a/Lib/test/test_email/test_policy.py	Sun Mar 09 11:18:16 2014 +0100
+++ b/Lib/test/test_email/test_policy.py	Sun Mar 09 23:42:42 2014 +0100
@@ -22,6 +22,7 @@
         'linesep':                  '\n',
         'cte_type':                 '8bit',
         'raise_on_defect':          False,
+        'mangle_from_':             True,
         }
     # These default values are the ones set on email.policy.default.
     # If any of these defaults change, the docs must be updated.
@@ -31,6 +32,7 @@
         'header_factory':           email.policy.EmailPolicy.header_factory,
         'refold_source':            'long',
         'content_manager':          email.policy.EmailPolicy.content_manager,
+        'mangle_from_':             False,
         })
 
     # For each policy under test, we give here what we expect the defaults to
@@ -39,7 +41,8 @@
     new_policy = email.policy.EmailPolicy()
     policies = {
         email.policy.compat32: make_defaults(compat32_defaults, {}),
-        email.policy.default: make_defaults(policy_defaults, {}),
+        email.policy.default: make_defaults(policy_defaults,
+                                            {'mangle_from_': False}),
         email.policy.SMTP: make_defaults(policy_defaults,
                                          {'linesep': '\r\n'}),
         email.policy.HTTP: make_defaults(policy_defaults,