diff --git a/Doc/library/mailbox.rst b/Doc/library/mailbox.rst
index da01791..ff8cfea 100644
--- a/Doc/library/mailbox.rst
+++ b/Doc/library/mailbox.rst
@@ -81,13 +81,16 @@
       it.
 
       Parameter *message* may be a :class:`Message` instance, an
-      :class:`email.Message.Message` instance, a string, or a file-like object
-      (which should be open in text mode). If *message* is an instance of the
+      :class:`email.Message.Message` instance, a string, a byte string, or a
+      file-like object (which should be open in binary mode). If *message* is
+      an instance of the
       appropriate format-specific :class:`Message` subclass (e.g., if it's an
       :class:`mboxMessage` instance and this is an :class:`mbox` instance), its
       format-specific information is used. Otherwise, reasonable defaults for
       format-specific information are used.
 
+      .. versionchanged:: 3.2 support for binary input
+
 
    .. method:: remove(key)
                __delitem__(key)
@@ -108,8 +111,9 @@
       :exc:`KeyError` exception if no message already corresponds to *key*.
 
       As with :meth:`add`, parameter *message* may be a :class:`Message`
-      instance, an :class:`email.Message.Message` instance, a string, or a
-      file-like object (which should be open in text mode). If *message* is an
+      instance, an :class:`email.Message.Message` instance, a string, a byte
+      string, or a file-like object (which should be open in binary mode). If
+      *message* is an
       instance of the appropriate format-specific :class:`Message` subclass
       (e.g., if it's an :class:`mboxMessage` instance and this is an
       :class:`mbox` instance), its format-specific information is
@@ -171,10 +175,20 @@
       raise a :exc:`KeyError` exception if no such message exists.
 
 
+   .. method:: get_bytes(key)
+
+      Return a byte representation of the message corresponding to *key*, or
+      raise a :exc:`KeyError` exception if no such message exists.
+
+      .. versionadded:: 3.2
+
+
    .. method:: get_string(key)
 
       Return a string representation of the message corresponding to *key*, or
-      raise a :exc:`KeyError` exception if no such message exists.
+      raise a :exc:`KeyError` exception if no such message exists.  The
+      message is processed through :class:`email.message.Message` to
+      convert it to a 7bit clean representation.
 
 
    .. method:: get_file(key)
@@ -184,9 +198,11 @@
       file-like object behaves as if open in binary mode.  This file should be
       closed once it is no longer needed.
 
-      .. versionadded:: 3.2
-         The file-like object supports the context manager protocol, so that
-         you can use a :keyword:`with` statement to automatically close it.
+      .. versionchanged:: 3.2
+         The file object really is a binary file; previously it was incorrectly
+         returned in text mode.  Also, the file-like object now supports the
+         context manager protocol: you can use a :keyword:`with` statement to
+         automatically close it.
 
       .. note::
 
@@ -746,9 +762,11 @@
    If *message* is omitted, the new instance is created in a default, empty state.
    If *message* is an :class:`email.Message.Message` instance, its contents are
    copied; furthermore, any format-specific information is converted insofar as
-   possible if *message* is a :class:`Message` instance. If *message* is a string
+   possible if *message* is a :class:`Message` instance. If *message* is a string,
+   a byte string,
    or a file, it should contain an :rfc:`2822`\ -compliant message, which is read
-   and parsed.
+   and parsed.  Files should be open in binary mode, but text mode files
+   are accepted for backward compatibility.
 
    The format-specific state and behaviors offered by subclasses vary, but in
    general it is only the properties that are not specific to a particular
diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 520463a..53f4159 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -15,6 +15,7 @@
 import socket
 import errno
 import copy
+import warnings
 import email
 import email.message
 import email.generator
@@ -31,6 +32,8 @@
             'Message', 'MaildirMessage', 'mboxMessage', 'MHMessage',
             'BabylMessage', 'MMDFMessage']
 
+linesep = os.linesep.encode('ascii')
+
 class Mailbox:
     """A group of messages in a particular place."""
 
@@ -80,7 +83,14 @@
         raise NotImplementedError('Method must be implemented by subclass')
 
     def get_string(self, key):
-        """Return a string representation or raise a KeyError."""
+        """Return a string representation or raise a KeyError.
+
+        Uses email.message.Message to create a 7bit clean string
+        representation of the message."""
+        return email.message_from_bytes(self.get_bytes(key)).as_string()
+
+    def get_bytes(self, key):
+        """Return a byte string representation or raise a KeyError."""
         raise NotImplementedError('Method must be implemented by subclass')
 
     def get_file(self, key):
@@ -186,31 +196,55 @@
         """Flush and close the mailbox."""
         raise NotImplementedError('Method must be implemented by subclass')
 
+    def _string_to_bytes(self, message):
+        # If a message is not 7bit clean, we refuse to handle it since it
+        # likely came from reading invalid messages in text mode, and that way
+        # lies mojibake.
+        try:
+            return message.encode('ascii')
+        except UnicodeError:
+            raise ValueError("String input must be ASCII-only; "
+                "use bytes or a Message instead")
+
     def _dump_message(self, message, target, mangle_from_=False):
-        # This assumes the target file is open in *text* mode with the
-        # desired encoding and newline setting.
+        # This assumes the target file is open in binary mode.
         """Dump message contents to target file."""
         if isinstance(message, email.message.Message):
-            buffer = io.StringIO()
-            gen = email.generator.Generator(buffer, mangle_from_, 0)
+            buffer = io.BytesIO()
+            gen = email.generator.BytesGenerator(buffer, mangle_from_, 0)
             gen.flatten(message)
             buffer.seek(0)
             data = buffer.read()
-            ##data = data.replace('\n', os.linesep)
+            data = data.replace(b'\n', linesep)
             target.write(data)
-        elif isinstance(message, str):
+        elif isinstance(message, (str, bytes, io.StringIO)):
+            if isinstance(message, io.StringIO):
+                warnings.warn("Use of StringIO input is deprecated, "
+                    "use BytesIO instead", DeprecationWarning, 3)
+                message = message.getvalue()
+            if isinstance(message, str):
+                message = self._string_to_bytes(message)
             if mangle_from_:
-                message = message.replace('\nFrom ', '\n>From ')
-            ##message = message.replace('\n', os.linesep)
+                message = message.replace(b'\nFrom ', b'\n>From ')
+            message = message.replace(b'\n', linesep)
             target.write(message)
         elif hasattr(message, 'read'):
+            if hasattr(message, 'buffer'):
+                warnings.warn("Use of text mode files is deprecated, "
+                    "use a binary mode file instead", DeprecationWarning, 3)
+                message = message.buffer
             while True:
                 line = message.readline()
+                # Universal newline support.
+                if line.endswith(b'\r\n'):
+                    line = line[:-2] + b'\n'
+                elif line.endswith(b'\r'):
+                    line = line[:-1] + b'\n'
                 if not line:
                     break
-                if mangle_from_ and line.startswith('From '):
-                    line = '>From ' + line[5:]
-                ##line = line.replace('\n', os.linesep)
+                if mangle_from_ and line.startswith(b'From '):
+                    line = b'>From ' + line[5:]
+                line = line.replace(b'\n', linesep)
                 target.write(line)
         else:
             raise TypeError('Invalid message type: %s' % type(message))
@@ -319,7 +353,7 @@
     def get_message(self, key):
         """Return a Message representation or raise a KeyError."""
         subpath = self._lookup(key)
-        f = open(os.path.join(self._path, subpath), 'r', newline='')
+        f = open(os.path.join(self._path, subpath), 'rb')
         try:
             if self._factory:
                 msg = self._factory(f)
@@ -334,17 +368,17 @@
         msg.set_date(os.path.getmtime(os.path.join(self._path, subpath)))
         return msg
 
-    def get_string(self, key):
-        """Return a string representation or raise a KeyError."""
-        f = open(os.path.join(self._path, self._lookup(key)), 'r', newline='')
+    def get_bytes(self, key):
+        """Return a bytes representation or raise a KeyError."""
+        f = open(os.path.join(self._path, self._lookup(key)), 'rb')
         try:
-            return f.read()
+            return f.read().replace(linesep, b'\n')
         finally:
             f.close()
 
     def get_file(self, key):
         """Return a file-like representation or raise a KeyError."""
-        f = open(os.path.join(self._path, self._lookup(key)), 'r', newline='')
+        f = open(os.path.join(self._path, self._lookup(key)), 'rb')
         return _ProxyFile(f)
 
     def iterkeys(self):
@@ -534,15 +568,15 @@
         """Initialize a single-file mailbox."""
         Mailbox.__init__(self, path, factory, create)
         try:
-            f = open(self._path, 'r+', newline='')
+            f = open(self._path, 'rb+')
         except IOError as e:
             if e.errno == errno.ENOENT:
                 if create:
-                    f = open(self._path, 'w+', newline='')
+                    f = open(self._path, 'wb+')
                 else:
                     raise NoSuchMailboxError(self._path)
             elif e.errno == errno.EACCES:
-                f = open(self._path, 'r', newline='')
+                f = open(self._path, 'rb')
             else:
                 raise
         self._file = f
@@ -708,20 +742,25 @@
         """Return a Message representation or raise a KeyError."""
         start, stop = self._lookup(key)
         self._file.seek(start)
-        from_line = self._file.readline().replace(os.linesep, '')
+        from_line = self._file.readline().replace(linesep, b'')
         string = self._file.read(stop - self._file.tell())
-        msg = self._message_factory(string.replace(os.linesep, '\n'))
-        msg.set_from(from_line[5:])
+        msg = self._message_factory(string.replace(linesep, b'\n'))
+        msg.set_from(from_line[5:].decode('ascii'))
         return msg
 
     def get_string(self, key, from_=False):
         """Return a string representation or raise a KeyError."""
+        return email.message_from_bytes(
+            self.get_bytes(key)).as_string(unixfrom=from_)
+
+    def get_bytes(self, key, from_=False):
+        """Return a string representation or raise a KeyError."""
         start, stop = self._lookup(key)
         self._file.seek(start)
         if not from_:
             self._file.readline()
         string = self._file.read(stop - self._file.tell())
-        return string.replace(os.linesep, '\n')
+        return string.replace(linesep, b'\n')
 
     def get_file(self, key, from_=False):
         """Return a file-like representation or raise a KeyError."""
@@ -734,22 +773,27 @@
     def _install_message(self, message):
         """Format a message and blindly write to self._file."""
         from_line = None
-        if isinstance(message, str) and message.startswith('From '):
-            newline = message.find('\n')
+        if isinstance(message, str):
+            message = self._string_to_bytes(message)
+        if isinstance(message, bytes) and message.startswith(b'From '):
+            newline = message.find(b'\n')
             if newline != -1:
                 from_line = message[:newline]
                 message = message[newline + 1:]
             else:
                 from_line = message
-                message = ''
+                message = b''
         elif isinstance(message, _mboxMMDFMessage):
-            from_line = 'From ' + message.get_from()
+            author = message.get_from().encode('ascii')
+            from_line = b'From ' + author
         elif isinstance(message, email.message.Message):
             from_line = message.get_unixfrom()  # May be None.
+            if from_line is not None:
+                from_line = from_line.encode('ascii')
         if from_line is None:
-            from_line = 'From MAILER-DAEMON %s' % time.asctime(time.gmtime())
+            from_line = b'From MAILER-DAEMON ' + time.asctime(time.gmtime()).encode()
         start = self._file.tell()
-        self._file.write(from_line + os.linesep)
+        self._file.write(from_line + linesep)
         self._dump_message(message, self._file, self._mangle_from_)
         stop = self._file.tell()
         return (start, stop)
@@ -768,7 +812,7 @@
     def _pre_message_hook(self, f):
         """Called before writing each message to file f."""
         if f.tell() != 0:
-            f.write(os.linesep)
+            f.write(linesep)
 
     def _generate_toc(self):
         """Generate key-to-(start, stop) table of contents."""
@@ -777,9 +821,9 @@
         while True:
             line_pos = self._file.tell()
             line = self._file.readline()
-            if line.startswith('From '):
+            if line.startswith(b'From '):
                 if len(stops) < len(starts):
-                    stops.append(line_pos - len(os.linesep))
+                    stops.append(line_pos - len(linesep))
                 starts.append(line_pos)
             elif not line:
                 stops.append(line_pos)
@@ -799,11 +843,11 @@
 
     def _pre_message_hook(self, f):
         """Called before writing each message to file f."""
-        f.write('\001\001\001\001' + os.linesep)
+        f.write(b'\001\001\001\001' + linesep)
 
     def _post_message_hook(self, f):
         """Called after writing each message to file f."""
-        f.write(os.linesep + '\001\001\001\001' + os.linesep)
+        f.write(linesep + b'\001\001\001\001' + linesep)
 
     def _generate_toc(self):
         """Generate key-to-(start, stop) table of contents."""
@@ -814,14 +858,14 @@
             line_pos = next_pos
             line = self._file.readline()
             next_pos = self._file.tell()
-            if line.startswith('\001\001\001\001' + os.linesep):
+            if line.startswith(b'\001\001\001\001' + linesep):
                 starts.append(next_pos)
                 while True:
                     line_pos = next_pos
                     line = self._file.readline()
                     next_pos = self._file.tell()
-                    if line == '\001\001\001\001' + os.linesep:
-                        stops.append(line_pos - len(os.linesep))
+                    if line == b'\001\001\001\001' + linesep:
+                        stops.append(line_pos - len(linesep))
                         break
                     elif not line:
                         stops.append(line_pos)
@@ -890,7 +934,7 @@
         """Replace the keyed message; raise KeyError if it doesn't exist."""
         path = os.path.join(self._path, str(key))
         try:
-            f = open(path, 'r+', newline='')
+            f = open(path, 'rb+')
         except IOError as e:
             if e.errno == errno.ENOENT:
                 raise KeyError('No message with key: %s' % key)
@@ -914,9 +958,9 @@
         """Return a Message representation or raise a KeyError."""
         try:
             if self._locked:
-                f = open(os.path.join(self._path, str(key)), 'r+', newline='')
+                f = open(os.path.join(self._path, str(key)), 'rb+')
             else:
-                f = open(os.path.join(self._path, str(key)), 'r', newline='')
+                f = open(os.path.join(self._path, str(key)), 'rb')
         except IOError as e:
             if e.errno == errno.ENOENT:
                 raise KeyError('No message with key: %s' % key)
@@ -937,13 +981,13 @@
                 msg.add_sequence(name)
         return msg
 
-    def get_string(self, key):
-        """Return a string representation or raise a KeyError."""
+    def get_bytes(self, key):
+        """Return a bytes representation or raise a KeyError."""
         try:
             if self._locked:
-                f = open(os.path.join(self._path, str(key)), 'r+', newline='')
+                f = open(os.path.join(self._path, str(key)), 'rb+')
             else:
-                f = open(os.path.join(self._path, str(key)), 'r', newline='')
+                f = open(os.path.join(self._path, str(key)), 'rb')
         except IOError as e:
             if e.errno == errno.ENOENT:
                 raise KeyError('No message with key: %s' % key)
@@ -953,7 +997,7 @@
             if self._locked:
                 _lock_file(f)
             try:
-                return f.read()
+                return f.read().replace(linesep, b'\n')
             finally:
                 if self._locked:
                     _unlock_file(f)
@@ -963,7 +1007,7 @@
     def get_file(self, key):
         """Return a file-like representation or raise a KeyError."""
         try:
-            f = open(os.path.join(self._path, str(key)), 'r', newline='')
+            f = open(os.path.join(self._path, str(key)), 'rb')
         except IOError as e:
             if e.errno == errno.ENOENT:
                 raise KeyError('No message with key: %s' % key)
@@ -1041,7 +1085,7 @@
     def get_sequences(self):
         """Return a name-to-key-list dictionary to define each sequence."""
         results = {}
-        f = open(os.path.join(self._path, '.mh_sequences'), 'r', newline='')
+        f = open(os.path.join(self._path, '.mh_sequences'), 'r')
         try:
             all_keys = set(self.keys())
             for line in f:
@@ -1067,13 +1111,13 @@
 
     def set_sequences(self, sequences):
         """Set sequences using the given name-to-key-list dictionary."""
-        f = open(os.path.join(self._path, '.mh_sequences'), 'r+', newline='')
+        f = open(os.path.join(self._path, '.mh_sequences'), 'r+')
         try:
             os.close(os.open(f.name, os.O_WRONLY | os.O_TRUNC))
             for name, keys in sequences.items():
                 if len(keys) == 0:
                     continue
-                f.write('%s:' % name)
+                f.write(name + ':')
                 prev = None
                 completing = False
                 for key in sorted(set(keys)):
@@ -1168,50 +1212,55 @@
         """Return a Message representation or raise a KeyError."""
         start, stop = self._lookup(key)
         self._file.seek(start)
-        self._file.readline()   # Skip '1,' line specifying labels.
-        original_headers = io.StringIO()
+        self._file.readline()   # Skip b'1,' line specifying labels.
+        original_headers = io.BytesIO()
         while True:
             line = self._file.readline()
-            if line == '*** EOOH ***' + os.linesep or not line:
+            if line == b'*** EOOH ***' + linesep or not line:
                 break
-            original_headers.write(line.replace(os.linesep, '\n'))
-        visible_headers = io.StringIO()
+            original_headers.write(line.replace(linesep, b'\n'))
+        visible_headers = io.BytesIO()
         while True:
             line = self._file.readline()
-            if line == os.linesep or not line:
+            if line == linesep or not line:
                 break
-            visible_headers.write(line.replace(os.linesep, '\n'))
-        body = self._file.read(stop - self._file.tell()).replace(os.linesep,
-                                                                 '\n')
+            visible_headers.write(line.replace(linesep, b'\n'))
+        # Read up to the stop, or to the end
+        n = stop - self._file.tell()
+        assert n >= 0
+        body = self._file.read(n)
+        body = body.replace(linesep, b'\n')
         msg = BabylMessage(original_headers.getvalue() + body)
         msg.set_visible(visible_headers.getvalue())
         if key in self._labels:
             msg.set_labels(self._labels[key])
         return msg
 
-    def get_string(self, key):
+    def get_bytes(self, key):
         """Return a string representation or raise a KeyError."""
         start, stop = self._lookup(key)
         self._file.seek(start)
-        self._file.readline()   # Skip '1,' line specifying labels.
-        original_headers = io.StringIO()
+        self._file.readline()   # Skip b'1,' line specifying labels.
+        original_headers = io.BytesIO()
         while True:
             line = self._file.readline()
-            if line == '*** EOOH ***' + os.linesep or not line:
+            if line == b'*** EOOH ***' + linesep or not line:
                 break
-            original_headers.write(line.replace(os.linesep, '\n'))
+            original_headers.write(line.replace(linesep, b'\n'))
         while True:
             line = self._file.readline()
-            if line == os.linesep or not line:
+            if line == linesep or not line:
                 break
-        return original_headers.getvalue() + \
-               self._file.read(stop - self._file.tell()).replace(os.linesep,
-                                                                 '\n')
+        headers = original_headers.getvalue()
+        n = stop - self._file.tell()
+        assert n >= 0
+        data = self._file.read(n)
+        data = data.replace(linesep, b'\n')
+        return headers + data
 
     def get_file(self, key):
         """Return a file-like representation or raise a KeyError."""
-        return io.StringIO(self.get_string(key).replace('\n',
-                                                              os.linesep))
+        return io.BytesIO(self.get_bytes(key).replace(b'\n', linesep))
 
     def get_labels(self):
         """Return a list of user-defined labels in the mailbox."""
@@ -1232,19 +1281,19 @@
             line_pos = next_pos
             line = self._file.readline()
             next_pos = self._file.tell()
-            if line == '\037\014' + os.linesep:
+            if line == b'\037\014' + linesep:
                 if len(stops) < len(starts):
-                    stops.append(line_pos - len(os.linesep))
+                    stops.append(line_pos - len(linesep))
                 starts.append(next_pos)
                 labels = [label.strip() for label
-                                        in self._file.readline()[1:].split(',')
+                                        in self._file.readline()[1:].split(b',')
                                         if label.strip()]
                 label_lists.append(labels)
-            elif line == '\037' or line == '\037' + os.linesep:
+            elif line == b'\037' or line == b'\037' + linesep:
                 if len(stops) < len(starts):
-                    stops.append(line_pos - len(os.linesep))
+                    stops.append(line_pos - len(linesep))
             elif not line:
-                stops.append(line_pos - len(os.linesep))
+                stops.append(line_pos - len(linesep))
                 break
         self._toc = dict(enumerate(zip(starts, stops)))
         self._labels = dict(enumerate(label_lists))
@@ -1254,17 +1303,21 @@
 
     def _pre_mailbox_hook(self, f):
         """Called before writing the mailbox to file f."""
-        f.write('BABYL OPTIONS:%sVersion: 5%sLabels:%s%s\037' %
-                (os.linesep, os.linesep, ','.join(self.get_labels()),
-                 os.linesep))
+        babyl = b'BABYL OPTIONS:' + linesep
+        babyl += b'Version: 5' + linesep
+        labels = self.get_labels()
+        labels = (label.encode() for label in labels)
+        babyl += b'Labels:' + b','.join(labels) + linesep
+        babyl += b'\037'
+        f.write(babyl)
 
     def _pre_message_hook(self, f):
         """Called before writing each message to file f."""
-        f.write('\014' + os.linesep)
+        f.write(b'\014' + linesep)
 
     def _post_message_hook(self, f):
         """Called after writing each message to file f."""
-        f.write(os.linesep + '\037')
+        f.write(linesep + b'\037')
 
     def _install_message(self, message):
         """Write message contents and return (start, stop)."""
@@ -1277,68 +1330,80 @@
                     special_labels.append(label)
                 else:
                     labels.append(label)
-            self._file.write('1')
+            self._file.write(b'1')
             for label in special_labels:
-                self._file.write(', ' + label)
-            self._file.write(',,')
+                self._file.write(b', ' + label.encode())
+            self._file.write(b',,')
             for label in labels:
-                self._file.write(' ' + label + ',')
-            self._file.write(os.linesep)
+                self._file.write(b' ' + label.encode() + b',')
+            self._file.write(linesep)
         else:
-            self._file.write('1,,' + os.linesep)
+            self._file.write(b'1,,' + linesep)
         if isinstance(message, email.message.Message):
-            orig_buffer = io.StringIO()
-            orig_generator = email.generator.Generator(orig_buffer, False, 0)
+            orig_buffer = io.BytesIO()
+            orig_generator = email.generator.BytesGenerator(orig_buffer, False, 0)
             orig_generator.flatten(message)
             orig_buffer.seek(0)
             while True:
                 line = orig_buffer.readline()
-                self._file.write(line.replace('\n', os.linesep))
-                if line == '\n' or not line:
+                self._file.write(line.replace(b'\n', linesep))
+                if line == b'\n' or not line:
                     break
-            self._file.write('*** EOOH ***' + os.linesep)
+            self._file.write(b'*** EOOH ***' + linesep)
             if isinstance(message, BabylMessage):
-                vis_buffer = io.StringIO()
-                vis_generator = email.generator.Generator(vis_buffer, False, 0)
+                vis_buffer = io.BytesIO()
+                vis_generator = email.generator.BytesGenerator(vis_buffer, False, 0)
                 vis_generator.flatten(message.get_visible())
                 while True:
                     line = vis_buffer.readline()
-                    self._file.write(line.replace('\n', os.linesep))
-                    if line == '\n' or not line:
+                    self._file.write(line.replace(b'\n', linesep))
+                    if line == b'\n' or not line:
                         break
             else:
                 orig_buffer.seek(0)
                 while True:
                     line = orig_buffer.readline()
-                    self._file.write(line.replace('\n', os.linesep))
-                    if line == '\n' or not line:
+                    self._file.write(line.replace(b'\n', linesep))
+                    if line == b'\n' or not line:
                         break
             while True:
                 buffer = orig_buffer.read(4096) # Buffer size is arbitrary.
                 if not buffer:
                     break
-                self._file.write(buffer.replace('\n', os.linesep))
-        elif isinstance(message, str):
-            body_start = message.find('\n\n') + 2
+                self._file.write(buffer.replace(b'\n', linesep))
+        elif isinstance(message, (bytes, str, io.StringIO)):
+            if isinstance(message, io.StringIO):
+                warnings.warn("Use of StringIO input is deprecated, "
+                    "use BytesIO instead", DeprecationWarning, 3)
+                message = message.getvalue()
+            if isinstance(message, str):
+                message = self._string_to_bytes(message)
+            body_start = message.find(b'\n\n') + 2
             if body_start - 2 != -1:
-                self._file.write(message[:body_start].replace('\n',
-                                                              os.linesep))
-                self._file.write('*** EOOH ***' + os.linesep)
-                self._file.write(message[:body_start].replace('\n',
-                                                              os.linesep))
-                self._file.write(message[body_start:].replace('\n',
-                                                              os.linesep))
+                self._file.write(message[:body_start].replace(b'\n', linesep))
+                self._file.write(b'*** EOOH ***' + linesep)
+                self._file.write(message[:body_start].replace(b'\n', linesep))
+                self._file.write(message[body_start:].replace(b'\n', linesep))
             else:
-                self._file.write('*** EOOH ***' + os.linesep + os.linesep)
-                self._file.write(message.replace('\n', os.linesep))
+                self._file.write(b'*** EOOH ***' + linesep + linesep)
+                self._file.write(message.replace(b'\n', linesep))
         elif hasattr(message, 'readline'):
+            if hasattr(message, 'buffer'):
+                warnings.warn("Use of text mode files is deprecated, "
+                    "use a binary mode file instead", DeprecationWarning, 3)
+                message = message.buffer
             original_pos = message.tell()
             first_pass = True
             while True:
                 line = message.readline()
-                self._file.write(line.replace('\n', os.linesep))
-                if line == '\n' or not line:
-                    self._file.write('*** EOOH ***' + os.linesep)
+                # Universal newline support.
+                if line.endswith(b'\r\n'):
+                    line = line[:-2] + b'\n'
+                elif line.endswith(b'\r'):
+                    line = line[:-1] + b'\n'
+                self._file.write(line.replace(b'\n', linesep))
+                if line == b'\n' or not line:
+                    self._file.write(b'*** EOOH ***' + linesep)
                     if first_pass:
                         first_pass = False
                         message.seek(original_pos)
@@ -1348,7 +1413,7 @@
                 buffer = message.read(4096)     # Buffer size is arbitrary.
                 if not buffer:
                     break
-                self._file.write(buffer.replace('\n', os.linesep))
+                self._file.write(buffer.replace(b'\n', linesep))
         else:
             raise TypeError('Invalid message type: %s' % type(message))
         stop = self._file.tell()
@@ -1364,10 +1429,14 @@
             self._become_message(copy.deepcopy(message))
             if isinstance(message, Message):
                 message._explain_to(self)
+        elif isinstance(message, bytes):
+            self._become_message(email.message_from_bytes(message))
         elif isinstance(message, str):
             self._become_message(email.message_from_string(message))
-        elif hasattr(message, "read"):
+        elif isinstance(message, io.TextIOWrapper):
             self._become_message(email.message_from_file(message))
+        elif hasattr(message, "read"):
+            self._become_message(email.message_from_binary_file(message))
         elif message is None:
             email.message.Message.__init__(self)
         else:
@@ -1631,7 +1700,7 @@
             if not sequence in self._sequences:
                 self._sequences.append(sequence)
         else:
-            raise TypeError('sequence must be a string: %s' % type(sequence))
+            raise TypeError('sequence type must be str: %s' % type(sequence))
 
     def remove_sequence(self, sequence):
         """Remove sequence from the list of sequences including the message."""
@@ -1791,6 +1860,10 @@
         """Read bytes."""
         return self._read(size, self._file.read)
 
+    def read1(self, size=None):
+        """Read bytes."""
+        return self._read(size, self._file.read1)
+
     def readline(self, size=None):
         """Read a line."""
         return self._read(size, self._file.readline)
@@ -1847,6 +1920,22 @@
     def __exit__(self, *exc):
         self.close()
 
+    def readable(self):
+        return self._file.readable()
+
+    def writable(self):
+        return self._file.writable()
+
+    def seekable(self):
+        return self._file.seekable()
+
+    def flush(self):
+        return self._file.flush()
+
+    @property
+    def closed(self):
+        return self._file.closed
+
 
 class _PartialFile(_ProxyFile):
     """A read-only wrapper of part of a file."""
@@ -1875,7 +1964,7 @@
         """Read size bytes using read_method, honoring start and stop."""
         remaining = self._stop - self._pos
         if remaining <= 0:
-            return ''
+            return b''
         if size is None or size < 0 or size > remaining:
             size = remaining
         return _ProxyFile._read(self, size, read_method)
@@ -1942,7 +2031,7 @@
     """Create a file if it doesn't exist and open for reading and writing."""
     fd = os.open(path, os.O_CREAT | os.O_EXCL | os.O_RDWR, 0o666)
     try:
-        return open(path, 'r+', newline='')
+        return open(path, 'rb+')
     finally:
         os.close(fd)
 
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index de6d4bd..8e4c57a 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -7,8 +7,10 @@
 import email.message
 import re
 import io
+import tempfile
 from test import support
 import unittest
+import textwrap
 import mailbox
 import glob
 try:
@@ -48,6 +50,8 @@
 
 class TestMailbox(TestBase):
 
+    maxDiff = None
+
     _factory = None     # Overridden by subclasses to reuse tests
     _template = 'From: foo\n\n%s'
 
@@ -69,14 +73,108 @@
         self.assertEqual(len(self._box), 2)
         keys.append(self._box.add(email.message_from_string(_sample_message)))
         self.assertEqual(len(self._box), 3)
-        keys.append(self._box.add(io.StringIO(_sample_message)))
+        keys.append(self._box.add(io.BytesIO(_bytes_sample_message)))
         self.assertEqual(len(self._box), 4)
         keys.append(self._box.add(_sample_message))
         self.assertEqual(len(self._box), 5)
+        keys.append(self._box.add(_bytes_sample_message))
+        self.assertEqual(len(self._box), 6)
+        with self.assertWarns(DeprecationWarning):
+            keys.append(self._box.add(
+                io.TextIOWrapper(io.BytesIO(_bytes_sample_message))))
+        self.assertEqual(len(self._box), 7)
         self.assertEqual(self._box.get_string(keys[0]), self._template % 0)
-        for i in (1, 2, 3, 4):
+        for i in (1, 2, 3, 4, 5, 6):
             self._check_sample(self._box[keys[i]])
 
+    _nonascii_msg = textwrap.dedent("""\
+            From: foo
+            Subject: Falinaptár házhozszállítással. Már rendeltél?
+
+            0
+            """)
+
+    def test_add_invalid_8bit_bytes_header(self):
+        key = self._box.add(self._nonascii_msg.encode('latin1'))
+        self.assertEqual(len(self._box), 1)
+        self.assertEqual(self._box.get_bytes(key),
+            self._nonascii_msg.encode('latin1'))
+
+    def test_invalid_nonascii_header_as_string(self):
+        subj = self._nonascii_msg.splitlines()[1]
+        key = self._box.add(subj.encode('latin1'))
+        self.assertEqual(self._box.get_string(key),
+            'Subject: =?unknown-8bit?b?RmFsaW5hcHThciBo4Xpob3pzeuFsbO104XNz'
+            'YWwuIE3hciByZW5kZWx06Ww/?=\n\n')
+
+    def test_add_nonascii_header_raises(self):
+        with self.assertRaisesRegex(ValueError, "ASCII-only"):
+            self._box.add(self._nonascii_msg)
+
+    _non_latin_bin_msg = textwrap.dedent("""\
+        From: foo@bar.com
+        To: báz
+        Subject: Maintenant je vous présente mon collègue, le pouf célèbre
+        \tJean de Baddie
+        Mime-Version: 1.0
+        Content-Type: text/plain; charset="utf-8"
+        Content-Transfer-Encoding: 8bit
+
+        Да, они летят.
+        """).encode('utf-8')
+
+    def test_add_8bit_body(self):
+        key = self._box.add(self._non_latin_bin_msg)
+        self.assertEqual(self._box.get_bytes(key),
+                         self._non_latin_bin_msg)
+        with self._box.get_file(key) as f:
+            self.assertEqual(f.read(),
+                             self._non_latin_bin_msg.replace(b'\n',
+                                os.linesep.encode()))
+        self.assertEqual(self._box[key].get_payload(),
+                        "Да, они летят.\n")
+
+    def test_add_binary_file(self):
+        with tempfile.TemporaryFile('wb+') as f:
+            f.write(_bytes_sample_message)
+            f.seek(0)
+            key = self._box.add(f)
+        # See issue 11062
+        if not isinstance(self._box, mailbox.Babyl):
+            self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+                _bytes_sample_message.split(b'\n'))
+
+    def test_add_binary_nonascii_file(self):
+        with tempfile.TemporaryFile('wb+') as f:
+            f.write(self._non_latin_bin_msg)
+            f.seek(0)
+            key = self._box.add(f)
+        # See issue 11062
+        if not isinstance(self._box, mailbox.Babyl):
+            self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+                self._non_latin_bin_msg.split(b'\n'))
+
+    def test_add_text_file_warns(self):
+        with tempfile.TemporaryFile('w+') as f:
+            f.write(_sample_message)
+            f.seek(0)
+            with self.assertWarns(DeprecationWarning):
+                key = self._box.add(f)
+        # See issue 11062
+        if not isinstance(self._box, mailbox.Babyl):
+            self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+                _bytes_sample_message.split(b'\n'))
+
+    def test_add_StringIO_warns(self):
+        with self.assertWarns(DeprecationWarning):
+            key = self._box.add(io.StringIO(self._template % "0"))
+        self.assertEqual(self._box.get_string(key), self._template % "0")
+
+    def test_add_nonascii_StringIO_raises(self):
+        with self.assertWarns(DeprecationWarning):
+            with self.assertRaisesRegex(ValueError, "ASCII-only"):
+                self._box.add(io.StringIO(self._nonascii_msg))
+
     def test_remove(self):
         # Remove messages using remove()
         self._test_remove_or_delitem(self._box.remove)
@@ -154,12 +252,21 @@
         self.assertEqual(msg0.get_payload(), '0')
         self._check_sample(self._box.get_message(key1))
 
+    def test_get_bytes(self):
+        # Get bytes representations of messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(_sample_message)
+        self.assertEqual(self._box.get_bytes(key0),
+            (self._template % 0).encode('ascii'))
+        self.assertEqual(self._box.get_bytes(key1), _bytes_sample_message)
+
     def test_get_string(self):
         # Get string representations of messages
         key0 = self._box.add(self._template % 0)
         key1 = self._box.add(_sample_message)
         self.assertEqual(self._box.get_string(key0), self._template % 0)
-        self.assertEqual(self._box.get_string(key1), _sample_message)
+        self.assertEqual(self._box.get_string(key1).split('\n'),
+                         _sample_message.split('\n'))
 
     def test_get_file(self):
         # Get file representations of messages
@@ -169,9 +276,9 @@
             data0 = file.read()
         with self._box.get_file(key1) as file:
             data1 = file.read()
-        self.assertEqual(data0.replace(os.linesep, '\n'),
+        self.assertEqual(data0.decode('ascii').replace(os.linesep, '\n'),
                          self._template % 0)
-        self.assertEqual(data1.replace(os.linesep, '\n'),
+        self.assertEqual(data1.decode('ascii').replace(os.linesep, '\n'),
                          _sample_message)
 
     def test_iterkeys(self):
@@ -405,11 +512,12 @@
     def test_dump_message(self):
         # Write message representations to disk
         for input in (email.message_from_string(_sample_message),
-                      _sample_message, io.StringIO(_sample_message)):
-            output = io.StringIO()
+                      _sample_message, io.BytesIO(_bytes_sample_message)):
+            output = io.BytesIO()
             self._box._dump_message(input, output)
-            self.assertEqual(output.getvalue(), _sample_message)
-        output = io.StringIO()
+            self.assertEqual(output.getvalue(),
+                _bytes_sample_message.replace(b'\n', os.linesep.encode()))
+        output = io.BytesIO()
         self.assertRaises(TypeError,
                           lambda: self._box._dump_message(None, output))
 
@@ -439,6 +547,7 @@
         self.assertRaises(NotImplementedError, lambda: box.__getitem__(''))
         self.assertRaises(NotImplementedError, lambda: box.get_message(''))
         self.assertRaises(NotImplementedError, lambda: box.get_string(''))
+        self.assertRaises(NotImplementedError, lambda: box.get_bytes(''))
         self.assertRaises(NotImplementedError, lambda: box.get_file(''))
         self.assertRaises(NotImplementedError, lambda: '' in box)
         self.assertRaises(NotImplementedError, lambda: box.__contains__(''))
@@ -640,9 +749,9 @@
                              "Host name mismatch: '%s' should be '%s'" %
                              (groups[4], hostname))
             previous_groups = groups
-            tmp_file.write(_sample_message)
+            tmp_file.write(_bytes_sample_message)
             tmp_file.seek(0)
-            self.assertEqual(tmp_file.read(), _sample_message)
+            self.assertEqual(tmp_file.read(), _bytes_sample_message)
             tmp_file.close()
         file_count = len(os.listdir(os.path.join(self._path, "tmp")))
         self.assertEqual(file_count, repetitions,
@@ -787,6 +896,12 @@
         self.assertEqual(self._box[key].get_from(), 'foo@bar blah')
         self.assertEqual(self._box[key].get_payload(), '0')
 
+    def test_add_from_bytes(self):
+        # Add a byte string starting with 'From ' to the mailbox
+        key = self._box.add(b'From foo@bar blah\nFrom: foo\n\n0')
+        self.assertEqual(self._box[key].get_from(), 'foo@bar blah')
+        self.assertEqual(self._box[key].get_payload(), '0')
+
     def test_add_mbox_or_mmdf_message(self):
         # Add an mboxMessage or MMDFMessage
         for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
@@ -817,7 +932,7 @@
         self._box._file.seek(0)
         contents = self._box._file.read()
         self._box.close()
-        with open(self._path, 'r', newline='') as f:
+        with open(self._path, 'rb') as f:
             self.assertEqual(contents, f.read())
         self._box = self._factory(self._path)
 
@@ -1087,6 +1202,15 @@
             self._post_initialize_hook(msg)
             self._check_sample(msg)
 
+    def test_initialize_with_binary_file(self):
+        # Initialize based on contents of binary file
+        with open(self._path, 'wb+') as f:
+            f.write(_bytes_sample_message)
+            f.seek(0)
+            msg = self._factory(f)
+            self._post_initialize_hook(msg)
+            self._check_sample(msg)
+
     def test_initialize_with_nothing(self):
         # Initialize without arguments
         msg = self._factory()
@@ -1363,6 +1487,14 @@
             msg_plain = mailbox.Message(msg)
             self._check_sample(msg_plain)
 
+    def test_x_from_bytes(self):
+        # Convert all formats to Message
+        for class_ in (mailbox.Message, mailbox.MaildirMessage,
+                       mailbox.mboxMessage, mailbox.MHMessage,
+                       mailbox.BabylMessage, mailbox.MMDFMessage):
+            msg = class_(_bytes_sample_message)
+            self._check_sample(msg)
+
     def test_x_to_invalid(self):
         # Convert all formats to an invalid format
         for class_ in (mailbox.Message, mailbox.MaildirMessage,
@@ -1908,6 +2040,8 @@
 --NMuMz9nt05w80d4+--
 """
 
+_bytes_sample_message = _sample_message.encode('ascii')
+
 _sample_headers = {
     "Return-Path":"<gkj@gregorykjohnson.com>",
     "X-Original-To":"gkj+person@localhost",
diff --git a/Misc/NEWS b/Misc/NEWS
index 57b2440..3882a77 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -16,6 +16,12 @@
 Library
 -------
 
+- Issue #9124: mailbox now accepts binary input and reads and writes mailbox
+  files in binary mode, using the email package's binary support to parse
+  arbitrary email messages.  StringIO and text file input is deprecated,
+  and string input fails early if non-ASCII characters are used, where
+  previously it would fail when the email was processed in a later step.
+
 - Issue #10845: Mitigate the incompatibility between the multiprocessing
   module on Windows and the use of package, zipfile or directory execution
   by special casing main modules that actually *are* called __main__.py.