diff -r de81e0fe4905 Lib/email/_policybase.py
--- a/Lib/email/_policybase.py	Sun Mar 02 20:29:18 2014 +0100
+++ b/Lib/email/_policybase.py	Mon Mar 03 14:33:40 2014 +0100
@@ -155,6 +155,7 @@
     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 de81e0fe4905 Lib/email/generator.py
--- a/Lib/email/generator.py	Sun Mar 02 20:29:18 2014 +0100
+++ b/Lib/email/generator.py	Mon Mar 03 14:33:40 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 de81e0fe4905 Lib/email/policy.py
--- a/Lib/email/policy.py	Sun Mar 02 20:29:18 2014 +0100
+++ b/Lib/email/policy.py	Mon Mar 03 14:33:40 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