diff -r a47996c10579 Doc/library/smtpd.rst
--- a/Doc/library/smtpd.rst	Wed Aug 06 18:55:54 2014 +0300
+++ b/Doc/library/smtpd.rst	Sat Aug 09 00:38:23 2014 +0000
@@ -60,6 +60,15 @@
       argument will be a unicode string.  If it is set to ``False``, it
       will be a bytes object.
 
+   .. method:: validate_recipient_address(address)
+
+      Returns ``True``. Override this in a subclass to accept or deny a
+      recipient addresses conditionally.
+      *address* is a string of the form ``local-part@domain``. If this method
+      returns ``False`` on a given address :meth:`SMTPChannel.smtp_RCPT`
+      answers with ``b'554 <[address]>: Relay access denied.'``.
+      Use this method to avoid becoming an open relay.
+
    .. attribute:: channel_class
 
       Override this in subclasses to use a custom :class:`SMTPChannel` for
@@ -71,6 +80,9 @@
    .. versionchanged:: 3.5 the *decode_data* argument was added, and *localaddr*
       and *remoteaddr* may now contain IPv6 addresses.
 
+   .. versionadded:: 3.6
+      The :meth:`validate_recipient_address` method was added.
+
 
 DebuggingServer Objects
 -----------------------
diff -r a47996c10579 Lib/smtpd.py
--- a/Lib/smtpd.py	Wed Aug 06 18:55:54 2014 +0300
+++ b/Lib/smtpd.py	Sat Aug 09 00:38:23 2014 +0000
@@ -558,6 +558,9 @@
         if not address:
             self.push('501 Syntax: RCPT TO: <address>')
             return
+        if not self.smtp_server.validate_recipient_address(address):
+            self.push('554 <%s>: Relay access denied.' % address)
+            return
         self.rcpttos.append(address)
         print('recips:', self.rcpttos, file=DEBUGSTREAM)
         self.push('250 OK')
@@ -655,6 +658,13 @@
         """
         raise NotImplementedError
 
+    def validate_recipient_address(self, address):
+        """
+        Override this method to determine if the server should relay messages
+        to a given address. Accepts any recipient by default.
+        """
+        return True
+
 
 class DebuggingServer(SMTPServer):
     # Do something with the gathered message
diff -r a47996c10579 Lib/test/test_smtpd.py
--- a/Lib/test/test_smtpd.py	Wed Aug 06 18:55:54 2014 +0300
+++ b/Lib/test/test_smtpd.py	Sat Aug 09 00:38:23 2014 +0000
@@ -55,6 +55,11 @@
         with self.assertWarns(DeprecationWarning):
             smtpd.SMTPServer((support.HOST, 0), ('b', 0))
 
+    def test_validate_recipient_address(self):
+        server = smtpd.SMTPServer(
+            (support.HOST, 0), ('b', 0), decode_data=True)
+        self.assertTrue(server.validate_recipient_address('test@example.com'))
+
     def tearDown(self):
         asyncore.close_all()
         asyncore.socket = smtpd.socket = socket
@@ -80,6 +85,41 @@
         self.assertEqual(server.socket.family, socket.AF_INET)
 
 
+class SMTPDRestrictedServerTest(unittest.TestCase):
+    def setUp(self):
+        smtpd.socket = asyncore.socket = mock_socket
+        self.allowed_domains = ['example.com', 'spam.egg']
+
+        class RestrictedSMTPServer(DummyServer):
+            allowed_domains = self.allowed_domains
+
+            def validate_recipient_address(self, address):
+                if address.split('@')[1] in self.allowed_domains:
+                    return True
+                return False
+
+        self.server = RestrictedSMTPServer(
+            (support.HOST, 0), ('b', 0), decode_data=True)
+        conn, addr = self.server.accept()
+        self.channel = smtpd.SMTPChannel(
+            self.server, conn, addr, decode_data=True)
+
+    def write_line(self, line):
+        self.channel.socket.queue_recv(line)
+        self.channel.handle_read()
+
+    def test_validate_recipient_address_with_restriction(self):
+        self.write_line(b'EHLO example')
+        self.write_line(b'MAIL FROM: foo@example.com')
+
+        for address in ['spam@egg.foo', 'foo@spam.egg', 'test@example.com']:
+            self.write_line(('RCPT TO: %s' % address).encode('ascii'))
+            self.assertEqual(
+                self.channel.socket.last.decode('ascii').split(' ')[0],
+                "250" if address.split('@')[1] in self.allowed_domains
+                else "554")
+
+
 class SMTPDChannelTest(unittest.TestCase):
     def setUp(self):
         smtpd.socket = asyncore.socket = mock_socket