diff -r 5e978d499066 Doc/library/email.policy.rst
--- a/Doc/library/email.policy.rst	Wed Apr 23 08:39:02 2014 -0700
+++ b/Doc/library/email.policy.rst	Thu Apr 24 14:07:14 2014 +0200
@@ -187,6 +187,18 @@
       :const:`False` (the default), defects will be passed to the
       :meth:`register_defect` method.
 
+
+
+  .. attribute:: mangle_from\_
+
+    If :const:`True`, lines starting with *"From "* in the body are
+    escaped by putting a ``>`` in front of them. This parameter is used when
+    the message is being serialized by a generator.
+    Default: :const:`True`.
+
+       .. versionadded:: 3.5
+              The *mangle_from_* parameter.
+
    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 5e978d499066 Lib/email/_policybase.py
--- a/Lib/email/_policybase.py	Wed Apr 23 08:39:02 2014 -0700
+++ b/Lib/email/_policybase.py	Thu Apr 24 14:07:14 2014 +0200
@@ -149,12 +149,18 @@
                            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. This is used when the message is being
+                           serialized by a generator. 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 5e978d499066 Lib/email/generator.py
--- a/Lib/email/generator.py	Wed Apr 23 08:39:02 2014 -0700
+++ b/Lib/email/generator.py	Thu Apr 24 14:07:14 2014 +0200
@@ -32,16 +32,16 @@
     # 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
@@ -55,6 +55,9 @@
         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
@@ -446,7 +449,7 @@
     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 5e978d499066 Lib/email/policy.py
--- a/Lib/email/policy.py	Wed Apr 23 08:39:02 2014 -0700
+++ b/Lib/email/policy.py	Thu Apr 24 14:07:14 2014 +0200
@@ -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 5e978d499066 Lib/test/test_email/test_generator.py
--- a/Lib/test/test_email/test_generator.py	Wed Apr 23 08:39:02 2014 -0700
+++ b/Lib/test/test_email/test_generator.py	Thu Apr 24 14:07:14 2014 +0200
@@ -139,6 +139,21 @@
         g.flatten(msg, linesep='\n')
         self.assertEqual(s.getvalue(), self.typ(expected))
 
+    def test_set_mangle_from_via_policy(self):
+        source = textwrap.dedent("""\
+            Subject: test that
+             from is mangeld in the body!
+
+            From time to time I write a rhyme.
+            """)
+        for b in True, False:
+            expected = source.replace('From ', '>From ') if b else source
+            msg = self.msgmaker(self.typ(source))
+            s = self.ioclass()
+            g = self.genclass(s, policy=policy.EmailPolicy(mangle_from_=b))
+            g.flatten(msg)
+            self.assertEqual(s.getvalue(), self.typ(expected))
+
 
 class TestGenerator(TestGeneratorBase, TestEmailBase):
 
diff -r 5e978d499066 Lib/test/test_email/test_policy.py
--- a/Lib/test/test_email/test_policy.py	Wed Apr 23 08:39:02 2014 -0700
+++ b/Lib/test/test_email/test_policy.py	Thu Apr 24 14:07:14 2014 +0200
@@ -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