diff --git a/Doc/library/email.message.rst b/Doc/library/email.message.rst
index 2c7be8b..7a74220 100644
--- a/Doc/library/email.message.rst
+++ b/Doc/library/email.message.rst
@@ -31,19 +31,32 @@
 Here are the methods of the :class:`Message` class:
 
 
-.. class:: Message()
+.. class:: Message(policy=compat32)
 
-   The constructor takes no arguments.
+   The *policy* argument determiens the :mod:`~email.policy` that will be used
+   to update the message model.  The default value, :class:`compat32
+   <email.policy.Compat32>` maintains backward compatibility with the
+   Python 3.2 version of the email package.  For more information see the
+   :mod:`~email.policy` documentation.
+
+   .. versionchanged:: 3.3 The *policy* keyword argument was added.
 
 
-   .. method:: as_string(unixfrom=False, maxheaderlen=0)
+   .. method:: as_string(unixfrom=False, maxheaderlen=0, policy=None)
 
       Return the entire message flattened as a string.  When optional *unixfrom*
-      is ``True``, the envelope header is included in the returned string.
-      *unixfrom* defaults to ``False``.  Flattening the message may trigger
-      changes to the :class:`Message` if defaults need to be filled in to
-      complete the transformation to a string (for example, MIME boundaries may
-      be generated or modified).
+      is true, the envelope header is included in the returned string.
+      *unixfrom* defaults to ``False``.  For backward compabitility reasons,
+      *maxheaderlen* defaults to ``0``, so if you want a different value you
+      must override it explicitly (the value specified for *max_line_length* in
+      the policy will be ignored by this method).  The *policy* argument may be
+      used to override the default policy obtained from the message instance.
+      This can be used to control some of the formatting produced by the
+      method, since the specified *policy* will be passed to the ``Generator``.
+
+      Flattening the message may trigger changes to the :class:`Message` if
+      defaults need to be filled in to complete the transformation to a string
+      (for example, MIME boundaries may be generated or modified).
 
       Note that this method is provided as a convenience and may not always
       format the message the way you want.  For example, by default it does
@@ -59,10 +72,57 @@
          g.flatten(msg)
          text = fp.getvalue()
 
+      If the message object contains binary data that is not encoded according
+      to RFC standards, the non-compliant data will be replaced by unicode
+      "unknown character" code points.  (See also :meth:`.as_bytes` and
+      :class:`~email.generator.BytesGenerator`.)
+
+      .. versionchanged:: 3.4 the *policy* keyword argument was added.
+
 
    .. method:: __str__()
 
-      Equivalent to ``as_string(unixfrom=True)``.
+      Equivalent to :meth:`.as_string()`.  Allows ``str(msg)`` to produce a
+      string containing the formatted message.
+
+
+   .. method:: as_bytes(unixfrom=False, policy=None)
+
+      Return the entire message flattened as a bytes object.  When optional
+      *unixfrom* is true, the envelope header is included in the returned
+      string.  *unixfrom* defaults to ``False``.  The *policy* argument may be
+      used to override the default policy obtained from the message instance.
+      This can be used to control some of the formatting produced by the
+      method, since the specified *policy* will be passed to the
+      ``BytesGenerator``.
+
+      Flattening the message may trigger changes to the :class:`Message` if
+      defaults need to be filled in to complete the transformation to a string
+      (for example, MIME boundaries may be generated or modified).
+
+      Note that this method is provided as a convenience and may not always
+      format the message the way you want.  For example, by default it does
+      not do the mangling of lines that begin with ``From`` that is
+      required by the unix mbox format.  For more flexibility, instantiate a
+      :class:`~email.generator.BytesGenerator` instance and use its
+      :meth:`flatten` method directly.  For example::
+
+         from io import BytesIO
+         from email.generator import BytesGenerator
+         fp = BytesIO()
+         g = BytesGenerator(fp, mangle_from_=True, maxheaderlen=60)
+         g.flatten(msg)
+         text = fp.getvalue()
+
+      .. versionadded:: 3.4
+
+
+   .. method:: __bytes__()
+
+      Equivalent to :meth:`.as_bytes()`.  Allows ``bytes(msg)`` to produce a
+      bytes object containing the formatted message.
+
+      .. versionadded:: 3.4
 
 
    .. method:: is_multipart()
diff --git a/Doc/library/email.policy.rst b/Doc/library/email.policy.rst
index 54ebb1c..93c5350 100644
--- a/Doc/library/email.policy.rst
+++ b/Doc/library/email.policy.rst
@@ -105,7 +105,8 @@
 
    >>> import os
    >>> with open('converted.txt', 'wb') as f:
-   ...     f.write(msg.as_string(policy=msg.policy.clone(linesep=os.linesep)))
+   ...     f.write(msg.as_bytes(policy=msg.policy.clone(linesep=os.linesep)))
+   17
 
 Policy objects can also be combined using the addition operator, producing a
 policy object whose settings are a combination of the non-default values of the
diff --git a/Doc/whatsnew/3.4.rst b/Doc/whatsnew/3.4.rst
index 80b4b01..6011f8e 100644
--- a/Doc/whatsnew/3.4.rst
+++ b/Doc/whatsnew/3.4.rst
@@ -195,6 +195,26 @@
 plain tuple.  (Contributed by Claudiu Popa in :issue:`17818`.)
 
 
+email
+-----
+
+:meth:`~email.message.Message.as_string` now accepts a *policy* argument to
+override the default policy of the message when generating a string
+representation of it.  This means that ``as_string`` can now be used in more
+circumstances, instead of having to create and use a :mod:`~email.generator` in
+order to pass formatting parameters to its ``flatten`` method.
+
+New method :meth:`~email.message.Message.as_bytes` added to produce a bytes
+representation of the message in a fashion similar to how ``as_string``
+produces a string representation.  It does not accept the *maxheaderlen*
+argument, but does accept the *unixfrom* and *policy* arguments. The
+:class:`~email.message.Message` :meth:`~email.message.Message.__bytes__` method
+calls it, meaning that ``bytes(mymsg)`` will now produce the intuitive
+result:  a bytes object containing the fully formatted message.
+
+(Contributed by R. David Murray in :issue:`18600`.)
+
+
 functools
 ---------
 
diff --git a/Lib/email/message.py b/Lib/email/message.py
index 3feab52..b5f7b3a 100644
--- a/Lib/email/message.py
+++ b/Lib/email/message.py
@@ -132,22 +132,50 @@
 
     def __str__(self):
         """Return the entire formatted message as a string.
-        This includes the headers, body, and envelope header.
         """
         return self.as_string()
 
-    def as_string(self, unixfrom=False, maxheaderlen=0):
+    def as_string(self, unixfrom=False, maxheaderlen=0, policy=None):
         """Return the entire formatted message as a string.
-        Optional `unixfrom' when True, means include the Unix From_ envelope
-        header.
 
-        This is a convenience method and may not generate the message exactly
-        as you intend.  For more flexibility, use the flatten() method of a
-        Generator instance.
+        Optional 'unixfrom', when true, means include the Unix From_ envelope
+        header.  For backward compatibility reasons, if maxheaderlen is
+        not specified it defaults to 0, so you must override it explicitly
+        if you want a different maxheaderlen.  'policy' is passed to the
+        Generator instance used to serialize the mesasge; if it is not
+        specified the policy associated with the message instance is used.
+
+        If the message object contains binary data that is not encoded
+        according to RFC standards, the non-compliant data will be replaced by
+        unicode "unknown character" code points.
         """
         from email.generator import Generator
+        policy = self.policy if policy is None else policy
         fp = StringIO()
-        g = Generator(fp, mangle_from_=False, maxheaderlen=maxheaderlen)
+        g = Generator(fp,
+                      mangle_from_=False,
+                      maxheaderlen=maxheaderlen,
+                      policy=policy)
+        g.flatten(self, unixfrom=unixfrom)
+        return fp.getvalue()
+
+    def __bytes__(self):
+        """Return the entire formatted message as a bytes object.
+        """
+        return self.as_bytes()
+
+    def as_bytes(self, unixfrom=False, policy=None):
+        """Return the entire formatted message as a bytes object.
+
+        Optional 'unixfrom', when true, means include the Unix From_ envelope
+        header.  'policy' is passed to the BytesGenerator instance used to
+        serialize the message; if not specified the policy associated with
+        the message instance is used.
+        """
+        from email.generator import BytesGenerator
+        policy = self.policy if policy is None else policy
+        fp = BytesIO()
+        g = BytesGenerator(fp, mangle_from_=False, policy=policy)
         g.flatten(self, unixfrom=unixfrom)
         return fp.getvalue()
 
diff --git a/Lib/test/test_email/test_email.py b/Lib/test/test_email/test_email.py
index 4c82952..3507b1e 100644
--- a/Lib/test/test_email/test_email.py
+++ b/Lib/test/test_email/test_email.py
@@ -249,15 +249,42 @@
         self.assertTrue('TO' in msg)
 
     def test_as_string(self):
-        eq = self.ndiffAssertEqual
         msg = self._msgobj('msg_01.txt')
         with openfile('msg_01.txt') as fp:
             text = fp.read()
-        eq(text, str(msg))
+        self.assertEqual(text, str(msg))
         fullrepr = msg.as_string(unixfrom=True)
         lines = fullrepr.split('\n')
         self.assertTrue(lines[0].startswith('From '))
-        eq(text, NL.join(lines[1:]))
+        self.assertEqual(text, NL.join(lines[1:]))
+
+    def test_as_string_policy(self):
+        msg = self._msgobj('msg_01.txt')
+        newpolicy = msg.policy.clone(linesep='\r\n')
+        fullrepr = msg.as_string(policy=newpolicy)
+        s = StringIO()
+        g = Generator(s, policy=newpolicy)
+        g.flatten(msg)
+        self.assertEqual(fullrepr, s.getvalue())
+
+    def test_as_bytes(self):
+        msg = self._msgobj('msg_01.txt')
+        with openfile('msg_01.txt', 'rb') as fp:
+            data = fp.read()
+        self.assertEqual(data, bytes(msg))
+        fullrepr = msg.as_bytes(unixfrom=True)
+        lines = fullrepr.split(b'\n')
+        self.assertTrue(lines[0].startswith(b'From '))
+        self.assertEqual(data, b'\n'.join(lines[1:]))
+
+    def test_as_bytes_policy(self):
+        msg = self._msgobj('msg_01.txt')
+        newpolicy = msg.policy.clone(linesep='\r\n')
+        fullrepr = msg.as_bytes(policy=newpolicy)
+        s = BytesIO()
+        g = BytesGenerator(s,policy=newpolicy)
+        g.flatten(msg)
+        self.assertEqual(fullrepr, s.getvalue())
 
     # test_headerregistry.TestContentTypeHeader.bad_params
     def test_bad_param(self):
diff --git a/Misc/NEWS b/Misc/NEWS
index 261402e..b0e392f 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -22,6 +22,9 @@
 Library
 -------
 
+- Issue #18600: Added policy argument to email.message.Message.as_string,
+  and as_bytes and __bytes__ methods to Message.
+
 - Issue #18671: Output more information when logging exceptions occur.
 
 - Issue #18621: Prevent the site module's patched builtins from keeping