diff -r 36550e4f9b4c Lib/email/generator.py
--- a/Lib/email/generator.py	Tue Sep 13 12:21:47 2016 +0000
+++ b/Lib/email/generator.py	Tue Sep 13 20:27:59 2016 -0400
@@ -86,6 +86,23 @@
         from the policy associated with the msg.
 
         """
+        # If this is a newly constructed message (there are no parser-derived
+        # headers) and we have at least one content- header, add the
+        # MIME-Version header.
+        add_version = False
+        if ('MIME-Version' not in msg
+                and all(hasattr(v, 'name') for n, v in msg.raw_items())
+                and any(x.lower().startswith('content-') for x in msg)):
+            add_version = True
+        try:
+            if add_version:
+                msg['MIME-Version'] = '1.0'
+            return self._flatten(msg, unixfrom, linesep)
+        finally:
+            if add_version:
+                del msg['MIME-Version']
+
+    def _flatten(self, msg, unixfrom=False, linesep=None):
         # We use the _XXX constants for operating on data that comes directly
         # from the msg, and _encoded_XXX constants for operating on data that
         # has already been converted (to bytes in the BytesGenerator) and
@@ -269,7 +286,7 @@
         for part in subparts:
             s = self._new_buffer()
             g = self.clone(s)
-            g.flatten(part, unixfrom=False, linesep=self._NL)
+            g._flatten(part, unixfrom=False, linesep=self._NL)
             msgtexts.append(s.getvalue())
         # BAW: What about boundaries that are wrapped in double-quotes?
         boundary = msg.get_boundary()
@@ -328,7 +345,7 @@
         for part in msg.get_payload():
             s = self._new_buffer()
             g = self.clone(s)
-            g.flatten(part, unixfrom=False, linesep=self._NL)
+            g._flatten(part, unixfrom=False, linesep=self._NL)
             text = s.getvalue()
             lines = text.split(self._encoded_NL)
             # Strip off the unnecessary trailing empty line
@@ -355,7 +372,7 @@
         # in that case we just emit the string body.
         payload = msg._payload
         if isinstance(payload, list):
-            g.flatten(msg.get_payload(0), unixfrom=False, linesep=self._NL)
+            g._flatten(msg.get_payload(0), unixfrom=False, linesep=self._NL)
             payload = s.getvalue()
         else:
             payload = self._encode(payload)
diff -r 36550e4f9b4c Lib/email/message.py
--- a/Lib/email/message.py	Tue Sep 13 12:21:47 2016 +0000
+++ b/Lib/email/message.py	Tue Sep 13 20:27:59 2016 -0400
@@ -943,7 +943,7 @@
     from email.iterators import walk
 
 
-class MIMEPart(Message):
+class EmailMessage(Message):
 
     def __init__(self, policy=None):
         if policy is None:
@@ -1136,10 +1136,5 @@
                          if not n.lower().startswith('content-')]
         self._payload = None
 
-
-class EmailMessage(MIMEPart):
-
-    def set_content(self, *args, **kw):
-        super().set_content(*args, **kw)
-        if 'MIME-Version' not in self:
-            self['MIME-Version'] = '1.0'
+# Backward compatibility
+MIMEPart = EmailMessage
diff -r 36550e4f9b4c Lib/test/test_email/test_contentmanager.py
--- a/Lib/test/test_email/test_contentmanager.py	Tue Sep 13 12:21:47 2016 +0000
+++ b/Lib/test/test_email/test_contentmanager.py	Tue Sep 13 20:27:59 2016 -0400
@@ -297,6 +297,7 @@
         self.assertEqual(str(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
 
             Simple message.
             """))
@@ -310,6 +311,7 @@
         self.assertEqual(str(m), textwrap.dedent("""\
             Content-Type: text/html; charset="utf-8"
             Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
 
             <p>Simple message.</p>
             """))
@@ -323,6 +325,7 @@
         self.assertEqual(str(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="iso-8859-1"
             Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
 
             Simple message.
             """))
@@ -336,6 +339,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
 
             et là il est monté sur moi et il commence à m'éto.
             """).encode('utf-8'))
@@ -351,6 +355,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: quoted-printable
+            MIME-Version: 1.0
 
             j'ai un probl=C3=A8me de python. il est sorti de son vivari=
             um.  et l=C3=A0 il est mont=C3=A9 sur moi et il commence =
@@ -369,6 +374,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: quoted-printable
+            MIME-Version: 1.0
             """ + '\n'*10 + """
             j'ai un probl=C3=A8me de python. il est sorti de son vivari=
             um.  et l=C3=A0 il est mont=C3=A9 sur moi et il commence =
@@ -384,6 +390,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
 
             áàäéèęöő.
             """).encode('utf-8'))
@@ -397,6 +404,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
             """ + '\n'*10 + """
             áàäéèęöő.
             """).encode('utf-8'))
@@ -412,6 +420,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: base64
+            MIME-Version: 1.0
 
             w6HDoMOkw6nDqMSZw7bFkcOhw6DDpMOpw6jEmcO2xZHDocOgw6TDqcOoxJnD
             tsWRw6HDoMOkw6nDqMSZw7bFkcOhw6DDpMOpw6jEmcO2xZHDocOgw6TDqcOo
@@ -437,6 +446,7 @@
         self.assertEqual(bytes(m), textwrap.dedent("""\
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: quoted-printable
+            MIME-Version: 1.0
             """ + '\n'*10 + """
             =C3=A1=C3=A0=C3=A4=C3=A9=C3=A8=C4=99=C3=B6=C5=91=C3=A1=C3=
             =A0=C3=A4=C3=A9=C3=A8=C4=99=C3=B6=C5=91=C3=A1=C3=A0=C3=A4=
@@ -481,13 +491,13 @@
             Subject: Forwarded message
             Content-Type: message/rfc822
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
 
             To: python@vivarium.org
             From: police@monty.org
             Subject: get back in your box
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 7bit
-            MIME-Version: 1.0
 
             Or face the comfy chair.
             """))
@@ -511,13 +521,13 @@
             Subject: Escape report
             Content-Type: message/rfc822
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
 
             To: police@monty.org
             From: victim@monty.org
             Subject: Help
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: 8bit
-            MIME-Version: 1.0
 
             j'ai un problème de python. il est sorti de son vivarium.
             """).encode('utf-8'))
@@ -531,13 +541,13 @@
             Subject: Escape report
             Content-Type: message/rfc822
             Content-Transfer-Encoding: 8bit
+            MIME-Version: 1.0
 
             To: police@monty.org
             From: victim@monty.org
             Subject: Help
             Content-Type: text/plain; charset="utf-8"
             Content-Transfer-Encoding: base64
-            MIME-Version: 1.0
 
             aidhaSB1biBwcm9ibMOobWUgZGUgcHl0aG9uLiBpbCBlc3Qgc29ydGkgZGUgc29uIHZpdmFyaXVt
             Lgo=
@@ -575,6 +585,7 @@
                 self.assertEqual(str(m), textwrap.dedent("""\
                     Content-Type: image/jpeg
                     Content-Transfer-Encoding: base64
+                    MIME-Version: 1.0
 
                     Ym9ndXMgY29udGVudA==
                     """))
@@ -651,6 +662,7 @@
             From: foo@example.com
             Subject: I'm talking to myself.
             Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
 
             Simple message.
             """))
@@ -666,6 +678,7 @@
             X-Foo-Header: foo
             X-Bar-Header: bar
             Content-Transfer-Encoding: 7bit
+            MIME-Version: 1.0
 
             Simple message.
             """))
diff -r 36550e4f9b4c Lib/test/test_email/test_generator.py
--- a/Lib/test/test_email/test_generator.py	Tue Sep 13 12:21:47 2016 +0000
+++ b/Lib/test/test_email/test_generator.py	Tue Sep 13 20:27:59 2016 -0400
@@ -12,6 +12,7 @@
 class TestGeneratorBase:
 
     policy = policy.default
+    message = EmailMessage
 
     def msgmaker(self, msg, policy=None):
         policy = self.policy if policy is None else policy
@@ -143,7 +144,7 @@
     def test_set_mangle_from_via_policy(self):
         source = textwrap.dedent("""\
             Subject: test that
-             from is mangeld in the body!
+             from is mangled in the body!
 
             From time to time I write a rhyme.
             """)
@@ -162,6 +163,64 @@
                 g.flatten(msg)
                 self.assertEqual(s.getvalue(), self.typ(expected))
 
+    def test_mime_version_added_to_mime_message(self):
+        msg = self._make_message()
+        msg['To'] = 'foo@example.com'
+        msg.add_related('This is a test')
+        self.assertNotIn('MIME-Version', msg)
+        serialized = self.serialfunc(msg)
+        self.assertNotIn('MIME-Version', msg)
+        self.assertEqual(serialized, self.typ(textwrap.dedent("""\
+            To: foo@example.com
+            Content-Type: multipart/related; boundary="{boundary}"
+            MIME-Version: 1.0
+
+            --{boundary}
+            Content-Type: text/plain; charset="utf-8"
+            Content-Transfer-Encoding: 7bit
+            Content-Disposition: inline
+
+            This is a test
+
+            --{boundary}--
+            """).format(boundary=msg.get_boundary())))
+
+    def test_no_mime_version_in_non_mime_message(self):
+        msg = self._make_message()
+        msg['To'] = 'foo@example.com'
+        msg.set_payload('This is a test\n')
+        serialized = self.serialfunc(msg)
+        self.assertNotIn('MIME-Version', msg)
+        self.assertEqual(serialized, self.typ(textwrap.dedent("""\
+            To: foo@example.com
+
+            This is a test
+            """)))
+
+    def test_no_mime_version_added_to_parsed_message(self):
+        source = self.typ(textwrap.dedent("""\
+            Subject: test that broken message stays broken
+            Content-Type: text/plain
+
+            This message is missing a MIME-Version header.
+            """))
+        msg = self.msgmaker(source)
+        self.assertEqual(self.serialfunc(msg), source)
+        self.assertNotIn('MIME-Version', msg)
+
+    def test_no_mime_version_added_to_parsed_message_with_changed_header(self):
+        source = self.typ(textwrap.dedent("""\
+            Subject: test that broken message stays broken
+            Content-Type: text/plain
+            %s
+            This message is missing a MIME-Version header.
+            """))
+        msg = self.msgmaker(source % self.typ(''))
+        msg['To'] =  'bob@cat.com'
+        self.assertEqual(self.serialfunc(msg),
+                         source % self.typ('To: bob@cat.com\n'))
+        self.assertNotIn('MIME-Version', msg)
+
 
 class TestGenerator(TestGeneratorBase, TestEmailBase):
 
@@ -169,6 +228,7 @@
     genclass = Generator
     ioclass = io.StringIO
     typ = str
+    serialfunc = str
 
 
 class TestBytesGenerator(TestGeneratorBase, TestEmailBase):
@@ -177,6 +237,7 @@
     genclass = BytesGenerator
     ioclass = io.BytesIO
     typ = lambda self, x: x.encode('ascii')
+    serialfunc = bytes
 
     def test_cte_type_7bit_handles_unknown_8bit(self):
         source = ("Subject: Maintenant je vous présente mon "
diff -r 36550e4f9b4c Lib/test/test_email/test_message.py
--- a/Lib/test/test_email/test_message.py	Tue Sep 13 12:21:47 2016 +0000
+++ b/Lib/test/test_email/test_message.py	Tue Sep 13 20:27:59 2016 -0400
@@ -36,8 +36,8 @@
 
 
 @parameterize
-class TestEmailMessageBase:
-
+class TestEmailMessage(TestEmailBase):
+    message = EmailMessage
     policy = policy.default
 
     # The first argument is a triple (related, html, plain) of indices into the
@@ -743,41 +743,5 @@
         self.assertEqual(m.get_payload(), orig)
 
 
-class TestEmailMessage(TestEmailMessageBase, TestEmailBase):
-    message = EmailMessage
-
-    def test_set_content_adds_MIME_Version(self):
-        m = self._str_msg('')
-        cm = self._TestContentManager()
-        self.assertNotIn('MIME-Version', m)
-        m.set_content(content_manager=cm)
-        self.assertEqual(m['MIME-Version'], '1.0')
-
-    class _MIME_Version_adding_CM:
-        def set_content(self, msg, *args, **kw):
-            msg['MIME-Version'] = '1.0'
-
-    def test_set_content_does_not_duplicate_MIME_Version(self):
-        m = self._str_msg('')
-        cm = self._MIME_Version_adding_CM()
-        self.assertNotIn('MIME-Version', m)
-        m.set_content(content_manager=cm)
-        self.assertEqual(m['MIME-Version'], '1.0')
-
-
-class TestMIMEPart(TestEmailMessageBase, TestEmailBase):
-    # Doing the full test run here may seem a bit redundant, since the two
-    # classes are almost identical.  But what if they drift apart?  So we do
-    # the full tests so that any future drift doesn't introduce bugs.
-    message = MIMEPart
-
-    def test_set_content_does_not_add_MIME_Version(self):
-        m = self._str_msg('')
-        cm = self._TestContentManager()
-        self.assertNotIn('MIME-Version', m)
-        m.set_content(content_manager=cm)
-        self.assertNotIn('MIME-Version', m)
-
-
 if __name__ == '__main__':
     unittest.main()