diff --git a/Doc/library/smtpd.rst b/Doc/library/smtpd.rst
index c391f71..82f4763 100644
--- a/Doc/library/smtpd.rst
+++ b/Doc/library/smtpd.rst
@@ -20,17 +20,24 @@
 Additionally the SMTPChannel may be extended to implement very specific
 interaction behaviour with SMTP clients.
 
+The code supports :RFC:`5321`, plus the :rfc:`1870` SIZE extension.
+
+
 SMTPServer Objects
 ------------------
 
 
-.. class:: SMTPServer(localaddr, remoteaddr)
+.. class:: SMTPServer(localaddr, remoteaddr, data_size_limit=33554432)
 
    Create a new :class:`SMTPServer` object, which binds to local address
    *localaddr*.  It will treat *remoteaddr* as an upstream SMTP relayer.  It
    inherits from :class:`asyncore.dispatcher`, and so will insert itself into
    :mod:`asyncore`'s event loop on instantiation.
 
+   *data_size_limit* specifies the maximum number of bytes that will be
+   accepted in a ``DATA`` command.  A value of ``None`` or ``0`` means no
+   limit.
+
    .. method:: process_message(peer, mailfrom, rcpttos, data)
 
       Raise :exc:`NotImplementedError` exception. Override this in subclasses to
@@ -155,11 +162,15 @@
    Command  Action taken
    ======== ===================================================================
    HELO     Accepts the greeting from the client and stores it in
-            :attr:`seen_greeting`.
+            :attr:`seen_greeting`.  Sets server to base command mode.
+   EHLO     Accepts the greeting from the client and stores it in
+            :attr:`seen_greeting`.  Sets server to extended command mode.
    NOOP     Takes no action.
    QUIT     Closes the connection cleanly.
    MAIL     Accepts the "MAIL FROM:" syntax and stores the supplied address as
-            :attr:`mailfrom`.
+            :attr:`mailfrom`.  In extended command mode, accepts the
+            :rfc:`1870` SIZE attribute and responds appropriately based on the
+            value of ``data_size_limit``.
    RCPT     Accepts the "RCPT TO:" syntax and stores the supplied addresses in
             the :attr:`rcpttos` list.
    RSET     Resets the :attr:`mailfrom`, :attr:`rcpttos`, and
@@ -167,4 +178,7 @@
    DATA     Sets the internal state to :attr:`DATA` and stores remaining lines
             from the client in :attr:`received_data` until the terminator
             "\r\n.\r\n" is received.
+   HELP     Returns minimal information on command syntax
+   VRFY     Returns code 252 (the server doesn't know if the address is valid)
+   EXPN     Reports that the command is not implemented.
    ======== ===================================================================
diff --git a/Lib/smtpd.py b/Lib/smtpd.py
index 748fcae..778d6d6 100755
--- a/Lib/smtpd.py
+++ b/Lib/smtpd.py
@@ -1,5 +1,5 @@
 #! /usr/bin/env python3
-"""An RFC 2821 smtp proxy.
+"""An RFC 5321 smtp proxy.
 
 Usage: %(program)s [options] [localhost:localport [remotehost:remoteport]]
 
@@ -20,6 +20,11 @@
         Use `classname' as the concrete SMTP proxy class.  Uses `PureProxy' by
         default.
 
+    --size limit
+    -s limit
+        Restrict the total size of the incoming message to "limit" number of
+        bytes via the RFC 1870 SIZE extension.  Defaults to 33554432 bytes.
+
     --debug
     -d
         Turn on debugging prints.
@@ -35,10 +40,9 @@
 and if remoteport is not given, then 25 is used.
 """
 
-
 # Overview:
 #
-# This file implements the minimal SMTP protocol as defined in RFC 821.  It
+# This file implements the minimal SMTP protocol as defined in RFC 5321.  It
 # has a hierarchy of classes which implement the backend functionality for the
 # smtpd.  A number of classes are provided:
 #
@@ -66,7 +70,7 @@
 #
 # - support mailbox delivery
 # - alias files
-# - ESMTP
+# - Handle more ESMTP extensions
 # - handle error codes from the backend smtpd
 
 import sys
@@ -77,12 +81,14 @@
 import socket
 import asyncore
 import asynchat
+import collections
 from warnings import warn
+from email._header_value_parser import get_addr_spec, get_angle_addr
 
 __all__ = ["SMTPServer","DebuggingServer","PureProxy","MailmanProxy"]
 
 program = sys.argv[0]
-__version__ = 'Python SMTP proxy version 0.2'
+__version__ = 'Python SMTP proxy version 0.3'
 
 
 class Devnull:
@@ -94,9 +100,9 @@
 NEWLINE = '\n'
 EMPTYSTRING = ''
 COMMASPACE = ', '
+DATA_SIZE_DEFAULT = 33554432
 
 
-
 def usage(code, msg=''):
     print(__doc__ % globals(), file=sys.stderr)
     if msg:
@@ -104,19 +110,23 @@
     sys.exit(code)
 
 
-
 class SMTPChannel(asynchat.async_chat):
     COMMAND = 0
     DATA = 1
 
-    data_size_limit = 33554432
     command_size_limit = 512
+    command_size_limits = collections.defaultdict(lambda x=command_size_limit: x)
+    command_size_limits.update({
+        'MAIL': command_size_limit + 26,
+        })
+    max_command_size_limit = max(command_size_limits.values())
 
-    def __init__(self, server, conn, addr):
+    def __init__(self, server, conn, addr, data_size_limit=DATA_SIZE_DEFAULT):
         asynchat.async_chat.__init__(self, conn)
         self.smtp_server = server
         self.conn = conn
         self.addr = addr
+        self.data_size_limit = data_size_limit
         self.received_lines = []
         self.smtp_state = self.COMMAND
         self.seen_greeting = ''
@@ -137,6 +147,7 @@
         print('Peer:', repr(self.peer), file=DEBUGSTREAM)
         self.push('220 %s %s' % (self.fqdn, __version__))
         self.set_terminator(b'\r\n')
+        self.extended_smtp = False
 
     # properties for backwards-compatibility
     @property
@@ -268,7 +279,7 @@
     def collect_incoming_data(self, data):
         limit = None
         if self.smtp_state == self.COMMAND:
-            limit = self.command_size_limit
+            limit = self.max_command_size_limit
         elif self.smtp_state == self.DATA:
             limit = self.data_size_limit
         if limit and self.num_bytes > limit:
@@ -283,11 +294,7 @@
         print('Data:', repr(line), file=DEBUGSTREAM)
         self.received_lines = []
         if self.smtp_state == self.COMMAND:
-            if self.num_bytes > self.command_size_limit:
-                self.push('500 Error: line too long')
-                self.num_bytes = 0
-                return
-            self.num_bytes = 0
+            sz, self.num_bytes = self.num_bytes, 0
             if not line:
                 self.push('500 Error: bad syntax')
                 return
@@ -299,9 +306,14 @@
             else:
                 command = line[:i].upper()
                 arg = line[i+1:].strip()
+            max_sz = (self.command_size_limits[command]
+                        if self.extended_smtp else self.command_size_limit)
+            if sz > max_sz:
+                self.push('500 Error: line too long')
+                return
             method = getattr(self, 'smtp_' + command, None)
             if not method:
-                self.push('502 Error: command "%s" not implemented' % command)
+                self.push('500 Error: command "%s" not recognized' % command)
                 return
             method(arg)
             return
@@ -310,12 +322,12 @@
                 self.push('451 Internal confusion')
                 self.num_bytes = 0
                 return
-            if self.num_bytes > self.data_size_limit:
+            if self.data_size_limit and self.num_bytes > self.data_size_limit:
                 self.push('552 Error: Too much mail data')
                 self.num_bytes = 0
                 return
             # Remove extraneous carriage returns and de-transparency according
-            # to RFC 821, Section 4.5.2.
+            # to RFC 5321, Section 4.5.2.
             data = []
             for text in line.split('\r\n'):
                 if text and text[0] == '.':
@@ -333,7 +345,7 @@
             self.num_bytes = 0
             self.set_terminator(b'\r\n')
             if not status:
-                self.push('250 Ok')
+                self.push('250 OK')
             else:
                 self.push(status)
 
@@ -346,66 +358,188 @@
             self.push('503 Duplicate HELO/EHLO')
         else:
             self.seen_greeting = arg
+            self.extended_smtp = False
             self.push('250 %s' % self.fqdn)
 
+    def smtp_EHLO(self, arg):
+        if not arg:
+            self.push('501 Syntax: EHLO hostname')
+            return
+        if self.seen_greeting:
+            self.push('503 Duplicate HELO/EHLO')
+        else:
+            self.seen_greeting = arg
+            self.extended_smtp = True
+            self.push('250-%s' % self.fqdn)
+            if self.data_size_limit:
+                self.push('250-SIZE %s' % self.data_size_limit)
+            self.push('250 HELP')
+
     def smtp_NOOP(self, arg):
         if arg:
             self.push('501 Syntax: NOOP')
         else:
-            self.push('250 Ok')
+            self.push('250 OK')
 
     def smtp_QUIT(self, arg):
         # args is ignored
         self.push('221 Bye')
         self.close_when_done()
 
-    # factored
-    def __getaddr(self, keyword, arg):
-        address = None
+    def _strip_command_keyword(self, keyword, arg):
         keylen = len(keyword)
         if arg[:keylen].upper() == keyword:
-            address = arg[keylen:].strip()
-            if not address:
-                pass
-            elif address[0] == '<' and address[-1] == '>' and address != '<>':
-                # Addresses can be in the form <person@dom.com> but watch out
-                # for null address, e.g. <>
-                address = address[1:-1]
-        return address
+            return arg[keylen:].strip()
+        return ''
+
+    def _getaddr(self, arg):
+        if not arg:
+            return '', ''
+        if arg.lstrip().startswith('<'):
+            address, rest = get_angle_addr(arg)
+        else:
+            address, rest = get_addr_spec(arg)
+        if not address:
+            return address, rest
+        return address.addr_spec, rest
+
+    def _getparams(self, params):
+        # Return any parameters that appear to be syntactically valid according
+        # to RFC 1869, ignore all others.  (Postel rule: accept what we can.)
+        params = [param.split('=', 1) for param in params.split()
+                                      if '=' in param]
+        return {k: v for k, v in params if k.isalnum()}
+
+    def smtp_HELP(self, arg):
+        if arg:
+            extended = ' [SP <mail parameters]'
+            lc_arg = arg.upper()
+            if lc_arg == 'EHLO':
+                self.push('250 Syntax: EHLO hostname')
+            elif lc_arg == 'HELO':
+                self.push('250 Syntax: HELO hostname')
+            elif lc_arg == 'MAIL':
+                msg = '250 Syntax: MAIL FROM: <address>'
+                if self.extended_smtp:
+                    msg += extended
+                self.push(msg)
+            elif lc_arg == 'RCPT':
+                msg = '250 Syntax: RCPT TO: <address>'
+                if self.extended_smtp:
+                    msg += extended
+                self.push(msg)
+            elif lc_arg == 'DATA':
+                self.push('250 Syntax: DATA')
+            elif lc_arg == 'RSET':
+                self.push('250 Syntax: RSET')
+            elif lc_arg == 'NOOP':
+                self.push('250 Syntax: NOOP')
+            elif lc_arg == 'QUIT':
+                self.push('250 Syntax: QUIT')
+            elif lc_arg == 'VRFY':
+                self.push('250 Syntax: VRFY <address>')
+            else:
+                self.push('501 Supported commands: EHLO HELO MAIL RCPT '
+                          'DATA RSET NOOP QUIT VRFY')
+        else:
+            self.push('250 Supported commands: EHLO HELO MAIL RCPT DATA '
+                      'RSET NOOP QUIT VRFY')
+
+    def smtp_VRFY(self, arg):
+        if arg:
+            address, params = self._getaddr(arg)
+            if address:
+                self.push('252 Cannot VRFY user, but will accept message '
+                          'and attempt delivery')
+            else:
+                self.push('502 Could not VRFY %s' % arg)
+        else:
+            self.push('501 Syntax: VRFY <address>')
 
     def smtp_MAIL(self, arg):
         if not self.seen_greeting:
             self.push('503 Error: send HELO first');
             return
-
         print('===> MAIL', arg, file=DEBUGSTREAM)
-        address = self.__getaddr('FROM:', arg) if arg else None
+        syntaxerr = '501 Syntax: MAIL FROM: <address>'
+        if self.extended_smtp:
+            syntaxerr += ' [SP <mail-parameters>]'
+        if arg is None:
+            self.push(syntaxerr)
+            return
+        arg = self._strip_command_keyword('FROM:', arg)
+        address, params = self._getaddr(arg)
         if not address:
-            self.push('501 Syntax: MAIL FROM:<address>')
+            self.push(syntaxerr)
+            return
+        if not self.extended_smtp and params:
+            self.push(syntaxerr)
+            return
+        if not address:
+            self.push(syntaxerr)
             return
         if self.mailfrom:
             self.push('503 Error: nested MAIL command')
             return
+        params = self._getparams(params.upper())
+        if params is None:
+            self.push(syntaxerr)
+            return
+        size = params.pop('SIZE', None)
+        if size:
+            if not size.isdigit():
+                self.push(syntaxerr)
+                return
+            elif self.data_size_limit and int(size) > self.data_size_limit:
+                self.push('552 Error: message size exceeds fixed maximum message size')
+                return
+        if len(params.keys()) > 0:
+            self.push('555 MAIL FROM parameters not recognized or not implemented')
+            return
         self.mailfrom = address
         print('sender:', self.mailfrom, file=DEBUGSTREAM)
-        self.push('250 Ok')
+        self.push('250 OK')
 
     def smtp_RCPT(self, arg):
         if not self.seen_greeting:
             self.push('503 Error: send HELO first');
             return
-
         print('===> RCPT', arg, file=DEBUGSTREAM)
         if not self.mailfrom:
             self.push('503 Error: need MAIL command')
             return
-        address = self.__getaddr('TO:', arg) if arg else None
+        syntaxerr = '501 Syntax: RCPT TO: <address>'
+        if self.extended_smtp:
+            syntaxerr += ' [SP <mail-parameters>]'
+        if arg is None:
+            self.push(syntaxerr)
+            return
+        arg = self._strip_command_keyword('TO:', arg)
+        address, params = self._getaddr(arg)
+        if not address:
+            self.push(syntaxerr)
+            return
+        if params:
+            if self.extended_smtp:
+                params = self._getparams(params.upper())
+                if params is None:
+                    self.push(syntaxerr)
+                    return
+            else:
+                self.push(syntaxerr)
+                return
+        if not address:
+            self.push(syntaxerr)
+            return
+        if params and len(params.keys()) > 0:
+            self.push('555 RCPT TO parameters not recognized or not implemented')
+            return
         if not address:
             self.push('501 Syntax: RCPT TO: <address>')
             return
         self.rcpttos.append(address)
         print('recips:', self.rcpttos, file=DEBUGSTREAM)
-        self.push('250 Ok')
+        self.push('250 OK')
 
     def smtp_RSET(self, arg):
         if arg:
@@ -416,13 +550,12 @@
         self.rcpttos = []
         self.received_data = ''
         self.smtp_state = self.COMMAND
-        self.push('250 Ok')
+        self.push('250 OK')
 
     def smtp_DATA(self, arg):
         if not self.seen_greeting:
             self.push('503 Error: send HELO first');
             return
-
         if not self.rcpttos:
             self.push('503 Error: need RCPT command')
             return
@@ -433,15 +566,20 @@
         self.set_terminator(b'\r\n.\r\n')
         self.push('354 End data with <CR><LF>.<CR><LF>')
 
+    # Commands that have not been implemented
+    def smtp_EXPN(self, arg):
+        self.push('502 EXPN not implemented')
 
-
+
 class SMTPServer(asyncore.dispatcher):
     # SMTPChannel class to use for managing client connections
     channel_class = SMTPChannel
 
-    def __init__(self, localaddr, remoteaddr):
+    def __init__(self, localaddr, remoteaddr,
+                 data_size_limit=DATA_SIZE_DEFAULT):
         self._localaddr = localaddr
         self._remoteaddr = remoteaddr
+        self.data_size_limit = data_size_limit
         asyncore.dispatcher.__init__(self)
         try:
             self.create_socket(socket.AF_INET, socket.SOCK_STREAM)
@@ -459,7 +597,7 @@
 
     def handle_accepted(self, conn, addr):
         print('Incoming connection from %s' % repr(addr), file=DEBUGSTREAM)
-        channel = self.channel_class(self, conn, addr)
+        channel = self.channel_class(self, conn, addr, self.data_size_limit)
 
     # API for "doing something useful with the message"
     def process_message(self, peer, mailfrom, rcpttos, data):
@@ -487,7 +625,6 @@
         raise NotImplementedError
 
 
-
 class DebuggingServer(SMTPServer):
     # Do something with the gathered message
     def process_message(self, peer, mailfrom, rcpttos, data):
@@ -503,7 +640,6 @@
         print('------------ END MESSAGE ------------')
 
 
-
 class PureProxy(SMTPServer):
     def process_message(self, peer, mailfrom, rcpttos, data):
         lines = data.split('\n')
@@ -544,7 +680,6 @@
         return refused
 
 
-
 class MailmanProxy(PureProxy):
     def process_message(self, peer, mailfrom, rcpttos, data):
         from io import StringIO
@@ -623,19 +758,18 @@
                 msg.Enqueue(mlist, torequest=1)
 
 
-
 class Options:
     setuid = 1
     classname = 'PureProxy'
+    size_limit = None
 
 
-
 def parseargs():
     global DEBUGSTREAM
     try:
         opts, args = getopt.getopt(
-            sys.argv[1:], 'nVhc:d',
-            ['class=', 'nosetuid', 'version', 'help', 'debug'])
+            sys.argv[1:], 'nVhc:s:d',
+            ['class=', 'nosetuid', 'version', 'help', 'size=', 'debug'])
     except getopt.error as e:
         usage(1, e)
 
@@ -652,6 +786,13 @@
             options.classname = arg
         elif opt in ('-d', '--debug'):
             DEBUGSTREAM = sys.stderr
+        elif opt in ('-s', '--size'):
+            try:
+                int_size = int(arg)
+                options.size_limit = int_size
+            except:
+                print('Invalid size: ' + arg, file=sys.stderr)
+                sys.exit(1)
 
     # parse the rest of the arguments
     if len(args) < 1:
@@ -686,7 +827,6 @@
     return options
 
 
-
 if __name__ == '__main__':
     options = parseargs()
     # Become nobody
@@ -699,7 +839,8 @@
         import __main__ as mod
     class_ = getattr(mod, classname)
     proxy = class_((options.localhost, options.localport),
-                   (options.remotehost, options.remoteport))
+                   (options.remotehost, options.remoteport),
+                   options.size_limit)
     if options.setuid:
         try:
             import pwd
diff --git a/Lib/test/test_logging.py b/Lib/test/test_logging.py
index 3adeaec..26baf11 100644
--- a/Lib/test/test_logging.py
+++ b/Lib/test/test_logging.py
@@ -663,6 +663,7 @@
             self.smtp_server = server
             self.conn = conn
             self.addr = addr
+            self.data_size_limit = None
             self.received_lines = []
             self.smtp_state = self.COMMAND
             self.seen_greeting = ''
@@ -682,6 +683,7 @@
                 return
             self.push('220 %s %s' % (self.fqdn, smtpd.__version__))
             self.set_terminator(b'\r\n')
+            self.extended_smtp = False
 
 
     class TestSMTPServer(smtpd.SMTPServer):
@@ -709,6 +711,7 @@
         def __init__(self, addr, handler, poll_interval, sockmap):
             self._localaddr = addr
             self._remoteaddr = None
+            self.data_size_limit = None
             self.sockmap = sockmap
             asyncore.dispatcher.__init__(self, map=sockmap)
             try:
diff --git a/Lib/test/test_smtpd.py b/Lib/test/test_smtpd.py
index a7dc5f6..dda1941 100644
--- a/Lib/test/test_smtpd.py
+++ b/Lib/test/test_smtpd.py
@@ -1,4 +1,4 @@
-from unittest import TestCase
+import unittest
 from test import support, mock_socket
 import socket
 import io
@@ -26,7 +26,7 @@
         raise DummyDispatcherBroken()
 
 
-class SMTPDServerTest(TestCase):
+class SMTPDServerTest(unittest.TestCase):
     def setUp(self):
         smtpd.socket = asyncore.socket = mock_socket
 
@@ -39,7 +39,7 @@
             channel.socket.queue_recv(line)
             channel.handle_read()
 
-        write_line(b'HELO test.example')
+        write_line(b'HELO example')
         write_line(b'MAIL From:eggs@example')
         write_line(b'RCPT To:spam@example')
         write_line(b'DATA')
@@ -50,7 +50,7 @@
         asyncore.socket = smtpd.socket = socket
 
 
-class SMTPDChannelTest(TestCase):
+class SMTPDChannelTest(unittest.TestCase):
     def setUp(self):
         smtpd.socket = asyncore.socket = mock_socket
         self.old_debugstream = smtpd.DEBUGSTREAM
@@ -79,36 +79,94 @@
         self.assertEqual(self.channel.socket.last,
                          b'500 Error: bad syntax\r\n')
 
-    def test_EHLO_not_implemented(self):
-        self.write_line(b'EHLO test.example')
+    def test_EHLO(self):
+        self.write_line(b'EHLO example')
+        self.assertEqual(self.channel.socket.last, b'250 HELP\r\n')
+
+    def test_EHLO_bad_syntax(self):
+        self.write_line(b'EHLO')
         self.assertEqual(self.channel.socket.last,
-                         b'502 Error: command "EHLO" not implemented\r\n')
+                         b'501 Syntax: EHLO hostname\r\n')
+
+    def test_EHLO_duplicate(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'EHLO example')
+        self.assertEqual(self.channel.socket.last,
+                         b'503 Duplicate HELO/EHLO\r\n')
+
+    def test_EHLO_HELO_duplicate(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'HELO example')
+        self.assertEqual(self.channel.socket.last,
+                         b'503 Duplicate HELO/EHLO\r\n')
 
     def test_HELO(self):
         name = smtpd.socket.getfqdn()
-        self.write_line(b'HELO test.example')
+        self.write_line(b'HELO example')
         self.assertEqual(self.channel.socket.last,
                          '250 {}\r\n'.format(name).encode('ascii'))
 
+    def test_HELO_EHLO_duplicate(self):
+        self.write_line(b'HELO example')
+        self.write_line(b'EHLO example')
+        self.assertEqual(self.channel.socket.last,
+                         b'503 Duplicate HELO/EHLO\r\n')
+
+    def test_HELP(self):
+        self.write_line(b'HELP')
+        self.assertEqual(self.channel.socket.last,
+                         b'250 Supported commands: EHLO HELO MAIL RCPT ' + \
+                         b'DATA RSET NOOP QUIT VRFY\r\n')
+
+    def test_HELP_command(self):
+        self.write_line(b'HELP MAIL')
+        self.assertEqual(self.channel.socket.last,
+                         b'250 Syntax: MAIL FROM: <address>\r\n')
+
+    def test_HELP_command_unknown(self):
+        self.write_line(b'HELP SPAM')
+        self.assertEqual(self.channel.socket.last,
+                         b'501 Supported commands: EHLO HELO MAIL RCPT ' + \
+                         b'DATA RSET NOOP QUIT VRFY\r\n')
+
     def test_HELO_bad_syntax(self):
         self.write_line(b'HELO')
         self.assertEqual(self.channel.socket.last,
                          b'501 Syntax: HELO hostname\r\n')
 
     def test_HELO_duplicate(self):
-        self.write_line(b'HELO test.example')
-        self.write_line(b'HELO test.example')
+        self.write_line(b'HELO example')
+        self.write_line(b'HELO example')
         self.assertEqual(self.channel.socket.last,
                          b'503 Duplicate HELO/EHLO\r\n')
 
+    def test_HELO_parameter_rejected_when_extensions_not_enabled(self):
+        self.extended_smtp = False
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL from:<foo@example.com> SIZE=1234')
+        self.assertEqual(self.channel.socket.last,
+                         b'501 Syntax: MAIL FROM: <address>\r\n')
+
+    def test_MAIL_allows_space_after_colon(self):
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL from:   <foo@example.com>')
+        self.assertEqual(self.channel.socket.last,
+                         b'250 OK\r\n')
+
+    def test_extended_MAIL_allows_space_after_colon(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from:   <foo@example.com> size=20')
+        self.assertEqual(self.channel.socket.last,
+                         b'250 OK\r\n')
+
     def test_NOOP(self):
         self.write_line(b'NOOP')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
 
     def test_HELO_NOOP(self):
         self.write_line(b'HELO example')
         self.write_line(b'NOOP')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
 
     def test_NOOP_bad_syntax(self):
         self.write_line(b'NOOP hi')
@@ -136,15 +194,29 @@
 
     def test_command_too_long(self):
         self.write_line(b'HELO example')
-        self.write_line(b'MAIL from ' +
+        self.write_line(b'MAIL from: ' +
                         b'a' * self.channel.command_size_limit +
                         b'@example')
         self.assertEqual(self.channel.socket.last,
                          b'500 Error: line too long\r\n')
 
-    def test_data_too_long(self):
-        # Small hack. Setting limit to 2K octets here will save us some time.
-        self.channel.data_size_limit = 2048
+    def test_MAIL_command_limit_extended_with_SIZE(self):
+        self.write_line(b'EHLO example')
+        fill_len = self.channel.command_size_limit - len('MAIL from:<@example>')
+        self.write_line(b'MAIL from:<' +
+                        b'a' * fill_len +
+                        b'@example> SIZE=1234')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
+        self.write_line(b'MAIL from:<' +
+                        b'a' * (fill_len + 26) +
+                        b'@example> SIZE=1234')
+        self.assertEqual(self.channel.socket.last,
+                         b'500 Error: line too long\r\n')
+
+    def test_data_longer_than_default_data_size_limit(self):
+        # Hack the default so we don't have to generate so much data.
+        self.channel.data_size_limit = 1048
         self.write_line(b'HELO example')
         self.write_line(b'MAIL From:eggs@example')
         self.write_line(b'RCPT To:spam@example')
@@ -154,28 +226,93 @@
         self.assertEqual(self.channel.socket.last,
                          b'552 Error: Too much mail data\r\n')
 
+    def test_MAIL_size_parameter(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL FROM:<eggs@example> SIZE=512')
+        self.assertEqual(self.channel.socket.last,
+                         b'250 OK\r\n')
+
+    def test_MAIL_invalid_size_parameter(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL FROM:<eggs@example> SIZE=invalid')
+        self.assertEqual(self.channel.socket.last,
+            b'501 Syntax: MAIL FROM: <address> [SP <mail-parameters>]\r\n')
+
+    def test_MAIL_RCPT_unknown_parameters(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL FROM:<eggs@example> ham=green')
+        self.assertEqual(self.channel.socket.last,
+            b'555 MAIL FROM parameters not recognized or not implemented\r\n')
+
+        self.write_line(b'MAIL FROM:<eggs@example>')
+        self.write_line(b'RCPT TO:<eggs@example> ham=green')
+        self.assertEqual(self.channel.socket.last,
+            b'555 RCPT TO parameters not recognized or not implemented\r\n')
+
+    def test_MAIL_size_parameter_larger_than_default_data_size_limit(self):
+        self.channel.data_size_limit = 1048
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL FROM:<eggs@example> SIZE=2096')
+        self.assertEqual(self.channel.socket.last,
+            b'552 Error: message size exceeds fixed maximum message size\r\n')
+
     def test_need_MAIL(self):
         self.write_line(b'HELO example')
         self.write_line(b'RCPT to:spam@example')
         self.assertEqual(self.channel.socket.last,
             b'503 Error: need MAIL command\r\n')
 
-    def test_MAIL_syntax(self):
+    def test_MAIL_syntax_HELO(self):
         self.write_line(b'HELO example')
         self.write_line(b'MAIL from eggs@example')
         self.assertEqual(self.channel.socket.last,
-            b'501 Syntax: MAIL FROM:<address>\r\n')
+            b'501 Syntax: MAIL FROM: <address>\r\n')
 
-    def test_MAIL_missing_from(self):
+    def test_MAIL_syntax_EHLO(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from eggs@example')
+        self.assertEqual(self.channel.socket.last,
+            b'501 Syntax: MAIL FROM: <address> [SP <mail-parameters>]\r\n')
+
+    def test_MAIL_missing_address(self):
         self.write_line(b'HELO example')
         self.write_line(b'MAIL from:')
         self.assertEqual(self.channel.socket.last,
-            b'501 Syntax: MAIL FROM:<address>\r\n')
+            b'501 Syntax: MAIL FROM: <address>\r\n')
 
     def test_MAIL_chevrons(self):
         self.write_line(b'HELO example')
         self.write_line(b'MAIL from:<eggs@example>')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
+    def test_MAIL_empty_chevrons(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from:<>')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
+    def test_MAIL_quoted_localpart(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from: <"Fred Blogs"@example.com>')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.assertEqual(self.channel.mailfrom, '"Fred Blogs"@example.com')
+
+    def test_MAIL_quoted_localpart_no_angles(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from: "Fred Blogs"@example.com')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.assertEqual(self.channel.mailfrom, '"Fred Blogs"@example.com')
+
+    def test_MAIL_quoted_localpart_with_size(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from: <"Fred Blogs"@example.com> SIZE=1000')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.assertEqual(self.channel.mailfrom, '"Fred Blogs"@example.com')
+
+    def test_MAIL_quoted_localpart_with_size_no_angles(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL from: "Fred Blogs"@example.com SIZE=1000')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.assertEqual(self.channel.mailfrom, '"Fred Blogs"@example.com')
 
     def test_nested_MAIL(self):
         self.write_line(b'HELO example')
@@ -184,6 +321,22 @@
         self.assertEqual(self.channel.socket.last,
             b'503 Error: nested MAIL command\r\n')
 
+    def test_VRFY(self):
+        self.write_line(b'VRFY eggs@example')
+        self.assertEqual(self.channel.socket.last,
+            b'252 Cannot VRFY user, but will accept message and attempt ' + \
+            b'delivery\r\n')
+
+    def test_VRFY_syntax(self):
+        self.write_line(b'VRFY')
+        self.assertEqual(self.channel.socket.last,
+            b'501 Syntax: VRFY <address>\r\n')
+
+    def test_EXPN_not_implemented(self):
+        self.write_line(b'EXPN')
+        self.assertEqual(self.channel.socket.last,
+            b'502 EXPN not implemented\r\n')
+
     def test_no_HELO_MAIL(self):
         self.write_line(b'MAIL from:<foo@example.com>')
         self.assertEqual(self.channel.socket.last,
@@ -196,13 +349,26 @@
         self.assertEqual(self.channel.socket.last,
             b'503 Error: need RCPT command\r\n')
 
-    def test_RCPT_syntax(self):
+    def test_RCPT_syntax_HELO(self):
         self.write_line(b'HELO example')
-        self.write_line(b'MAIL From:eggs@example')
+        self.write_line(b'MAIL From: eggs@example')
         self.write_line(b'RCPT to eggs@example')
         self.assertEqual(self.channel.socket.last,
             b'501 Syntax: RCPT TO: <address>\r\n')
 
+    def test_RCPT_syntax_EHLO(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL From: eggs@example')
+        self.write_line(b'RCPT to eggs@example')
+        self.assertEqual(self.channel.socket.last,
+            b'501 Syntax: RCPT TO: <address> [SP <mail-parameters>]\r\n')
+
+    def test_RCPT_lowercase_to_OK(self):
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL From: eggs@example')
+        self.write_line(b'RCPT to: <eggs@example>')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
     def test_no_HELO_RCPT(self):
         self.write_line(b'RCPT to eggs@example')
         self.assertEqual(self.channel.socket.last,
@@ -211,15 +377,15 @@
     def test_data_dialog(self):
         self.write_line(b'HELO example')
         self.write_line(b'MAIL From:eggs@example')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
         self.write_line(b'RCPT To:spam@example')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
 
         self.write_line(b'DATA')
         self.assertEqual(self.channel.socket.last,
             b'354 End data with <CR><LF>.<CR><LF>\r\n')
         self.write_line(b'data\r\nmore\r\n.')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
         self.assertEqual(self.server.messages,
             [('peer', 'eggs@example', ['spam@example'], 'data\nmore')])
 
@@ -267,7 +433,7 @@
         self.write_line(b'MAIL From:eggs@example')
         self.write_line(b'RCPT To:spam@example')
         self.write_line(b'RSET')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
         self.write_line(b'MAIL From:foo@example')
         self.write_line(b'RCPT To:eggs@example')
         self.write_line(b'DATA')
@@ -278,12 +444,18 @@
     def test_HELO_RSET(self):
         self.write_line(b'HELO example')
         self.write_line(b'RSET')
-        self.assertEqual(self.channel.socket.last, b'250 Ok\r\n')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
 
     def test_RSET_syntax(self):
         self.write_line(b'RSET hi')
         self.assertEqual(self.channel.socket.last, b'501 Syntax: RSET\r\n')
 
+    def test_unknown_command(self):
+        self.write_line(b'UNKNOWN_CMD')
+        self.assertEqual(self.channel.socket.last,
+                         b'500 Error: command "UNKNOWN_CMD" not ' + \
+                         b'recognized\r\n')
+
     def test_attribute_deprecations(self):
         with support.check_warnings(('', DeprecationWarning)):
             spam = self.channel._SMTPChannel__server
@@ -330,8 +502,54 @@
         with support.check_warnings(('', DeprecationWarning)):
             self.channel._SMTPChannel__addr = 'spam'
 
-def test_main():
-    support.run_unittest(SMTPDServerTest, SMTPDChannelTest)
+
+class SMTPDChannelWithDataSizeLimitTest(unittest.TestCase):
+
+    def setUp(self):
+        smtpd.socket = asyncore.socket = mock_socket
+        self.debug = smtpd.DEBUGSTREAM = io.StringIO()
+        self.server = DummyServer('a', 'b')
+        conn, addr = self.server.accept()
+        # Set DATA size limit to 32 bytes for easy testing
+        self.channel = smtpd.SMTPChannel(self.server, conn, addr, 32)
+
+    def tearDown(self):
+        asyncore.close_all()
+        asyncore.socket = smtpd.socket = socket
+
+    def write_line(self, line):
+        self.channel.socket.queue_recv(line)
+        self.channel.handle_read()
+
+    def test_data_limit_dialog(self):
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL From:eggs@example')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.write_line(b'RCPT To:spam@example')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
+        self.write_line(b'DATA')
+        self.assertEqual(self.channel.socket.last,
+            b'354 End data with <CR><LF>.<CR><LF>\r\n')
+        self.write_line(b'data\r\nmore\r\n.')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.assertEqual(self.server.messages,
+            [('peer', 'eggs@example', ['spam@example'], 'data\nmore')])
+
+    def test_data_limit_dialog_too_much_data(self):
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL From:eggs@example')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+        self.write_line(b'RCPT To:spam@example')
+        self.assertEqual(self.channel.socket.last, b'250 OK\r\n')
+
+        self.write_line(b'DATA')
+        self.assertEqual(self.channel.socket.last,
+            b'354 End data with <CR><LF>.<CR><LF>\r\n')
+        self.write_line(b'This message is longer than 32 bytes\r\n.')
+        self.assertEqual(self.channel.socket.last,
+                         b'552 Error: Too much mail data\r\n')
+
 
 if __name__ == "__main__":
-    test_main()
+    unittest.main()
diff --git a/Lib/test/test_smtplib.py b/Lib/test/test_smtplib.py
index 18dde2f..befc49e 100644
--- a/Lib/test/test_smtplib.py
+++ b/Lib/test/test_smtplib.py
@@ -229,13 +229,13 @@
 
     def testNOOP(self):
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
-        expected = (250, b'Ok')
+        expected = (250, b'OK')
         self.assertEqual(smtp.noop(), expected)
         smtp.quit()
 
     def testRSET(self):
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
-        expected = (250, b'Ok')
+        expected = (250, b'OK')
         self.assertEqual(smtp.rset(), expected)
         smtp.quit()
 
@@ -246,10 +246,18 @@
         self.assertEqual(smtp.ehlo(), expected)
         smtp.quit()
 
-    def testVRFY(self):
-        # VRFY isn't implemented in DebuggingServer
+    def testNotImplemented(self):
+        # EXPN isn't implemented in DebuggingServer
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
-        expected = (502, b'Error: command "VRFY" not implemented')
+        expected = (502, b'EXPN not implemented')
+        smtp.putcmd('EXPN')
+        self.assertEqual(smtp.getreply(), expected)
+        smtp.quit()
+
+    def testVRFY(self):
+        smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
+        expected = (252, b'Cannot VRFY user, but will accept message ' + \
+                         b'and attempt delivery')
         self.assertEqual(smtp.vrfy('nobody@nowhere.com'), expected)
         self.assertEqual(smtp.verify('nobody@nowhere.com'), expected)
         smtp.quit()
@@ -265,7 +273,8 @@
 
     def testHELP(self):
         smtp = smtplib.SMTP(HOST, self.port, local_hostname='localhost', timeout=3)
-        self.assertEqual(smtp.help(), b'Error: command "HELP" not implemented')
+        self.assertEqual(smtp.help(), b'Supported commands: EHLO HELO MAIL ' + \
+                                      b'RCPT DATA RSET NOOP QUIT VRFY')
         smtp.quit()
 
     def testSend(self):
diff --git a/Misc/ACKS b/Misc/ACKS
index 16f55ea..371668f 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -112,6 +112,7 @@
 Matias Bordese
 Jurjen Bos
 Peter Bosch
+Dan Boswell
 Eric Bouck
 Thierry Bousch
 Sebastian Boving
@@ -494,6 +495,7 @@
 Jack Jansen
 Bill Janssen
 Thomas Jarosch
+Juhana Jauhiainen
 Zbigniew Jędrzejewski-Szmek
 Julien Jehannet
 Drew Jenkins
@@ -1039,6 +1041,7 @@
 Richard Townsend
 David Townshend
 Laurence Tratt
+Alberto Trevino
 Matthias Troffaes
 John Tromp
 Jason Trowbridge
diff --git a/Misc/NEWS b/Misc/NEWS
index 4f20446..50060e5 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -46,6 +46,9 @@
 Library
 -------
 
+- Issue #8739: Updated smtpd to support RFC 5321, and added support for the
+  RFC 1870 SIZE extension.
+
 - Issue #665194: Added a localtime function to email.utils to provide an
   aware local datetime for use in setting Date headers.