diff --git a/Lib/smtpd.py b/Lib/smtpd.py
index 8103ca9af0d7b3..7f1e626c758748 100755
--- a/Lib/smtpd.py
+++ b/Lib/smtpd.py
@@ -738,16 +738,17 @@ def __init__(self, *args, **kwargs):
             raise ValueError("PureProxy does not support SMTPUTF8.")
         super(PureProxy, self).__init__(*args, **kwargs)
 
-    def process_message(self, peer, mailfrom, rcpttos, data):
-        lines = data.split('\n')
+    def process_message(self, peer, mailfrom, rcpttos, data, **kwargs):
+        lines = data.split(b'\n')
         # Look for the last header
         i = 0
         for line in lines:
             if not line:
                 break
             i += 1
-        lines.insert(i, 'X-Peer: %s' % peer[0])
-        data = NEWLINE.join(lines)
+        peer_address = peer[0].encode('ascii')
+        lines.insert(i, b'X-Peer: %s' % peer_address)
+        data = b'\n'.join(lines)
         refused = self._deliver(mailfrom, rcpttos, data)
         # TBD: what to do with refused addresses?
         print('we got some refusals:', refused, file=DEBUGSTREAM)
@@ -783,7 +784,7 @@ def __init__(self, *args, **kwargs):
             raise ValueError("MailmanProxy does not support SMTPUTF8.")
         super(PureProxy, self).__init__(*args, **kwargs)
 
-    def process_message(self, peer, mailfrom, rcpttos, data):
+    def process_message(self, peer, mailfrom, rcpttos, data, **kwargs):
         from io import StringIO
         from Mailman import Utils
         from Mailman import Message
@@ -826,7 +827,7 @@ def process_message(self, peer, mailfrom, rcpttos, data):
             print('we got refusals:', refused, file=DEBUGSTREAM)
         # Now deliver directly to the list commands
         mlists = {}
-        s = StringIO(data)
+        s = StringIO(data.decode())
         msg = Message.Message(s)
         # These headers are required for the proper execution of Mailman.  All
         # MTAs in existence seem to add these if the original message doesn't
diff --git a/Lib/test/test_smtpd.py b/Lib/test/test_smtpd.py
index a9f7d5a3b8bc19..4c6fc605a8b1c3 100644
--- a/Lib/test/test_smtpd.py
+++ b/Lib/test/test_smtpd.py
@@ -5,6 +5,8 @@
 import io
 import smtpd
 import asyncore
+import smtplib
+from unittest.mock import MagicMock, patch
 
 
 class DummyServer(smtpd.SMTPServer):
@@ -160,6 +162,84 @@ def tearDown(self):
         asyncore.socket = smtpd.socket = socket
 
 
+class PureProxyServerTest(unittest.TestCase):
+
+    def setUp(self):
+        smtpd.socket = asyncore.socket = mock_socket
+
+    @patch('smtplib.SMTP')
+    def test_proxy_successful(self, mock_smtp):
+        fake_smtp = MagicMock()
+        fake_smtp.sendmail = MagicMock(return_value={})
+        mock_smtp.return_value = fake_smtp
+        server = smtpd.PureProxy((support.HOST, 0), ('b', 0))
+        conn, addr = server.accept()
+        channel = smtpd.SMTPChannel(server, conn, addr)
+
+        def write_line(line):
+            channel.socket.queue_recv(line)
+            channel.handle_read()
+
+        write_line(b'EHLO example')
+        write_line(b'MAIL From:eggs@example')
+        write_line(b'RCPT To:spam@example')
+        write_line(b'DATA')
+        write_line(b'From: test\n\nhello\n')
+        write_line(b'.')
+
+        mock_smtp.assert_called_once_with()
+        fake_smtp.connect.assert_called_once_with('b', 0)
+        fake_smtp.sendmail.assert_called_once_with(
+            'eggs@example',
+            ['spam@example'],
+            b'From: test\nX-Peer: peer-address\n\nhello\n'
+        )
+        fake_smtp.quit.assert_called_once_with()
+
+    @patch('smtplib.SMTP')
+    def test_deliver_refused(self, mock_smtp):
+        fake_smtp = MagicMock()
+        exc = smtplib.SMTPRecipientsRefused({'error@example': (1, 'testing')})
+        fake_smtp.sendmail = MagicMock(side_effect=exc)
+        mock_smtp.return_value = fake_smtp
+
+        server = smtpd.PureProxy((support.HOST, 0), ('b', 0))
+        refused = server._deliver(
+            'eggs@example',
+            ['spam@example'],
+            b'From: test\nX-Peer: peer-address\n\nhello\n'
+        )
+        self.assertEqual(refused, {'error@example': (1, 'testing')})
+        fake_smtp.connect.assert_called_once_with('b', 0)
+        fake_smtp.quit.assert_called_once_with()
+
+    @patch('smtplib.SMTP')
+    def test_deliver_error(self, mock_smtp):
+        fake_smtp = MagicMock()
+        exc = OSError()
+        exc.smtp_code = 42
+        fake_smtp.sendmail = MagicMock(side_effect=exc)
+        mock_smtp.return_value = fake_smtp
+
+        server = smtpd.PureProxy((support.HOST, 0), ('b', 0))
+        refused = server._deliver(
+            'eggs@example',
+            ['spam@example'],
+            b'From: test\nX-Peer: peer-address\n\nhello\n'
+        )
+        self.assertEqual(refused, {'spam@example': (42, 'ignore')})
+        fake_smtp.connect.assert_called_once_with('b', 0)
+        fake_smtp.quit.assert_called_once_with()
+
+    def test_init_enable_SMTPUTF8(self):
+        self.assertRaises(ValueError, smtpd.PureProxy,
+                          (support.HOST, 0), ('b', 0), enable_SMTPUTF8=True)
+
+    def tearDown(self):
+        asyncore.close_all()
+        asyncore.socket = smtpd.socket = socket
+
+
 class TestFamilyDetection(unittest.TestCase):
     def setUp(self):
         smtpd.socket = asyncore.socket = mock_socket
diff --git a/Misc/ACKS b/Misc/ACKS
index 81b51f75199159..ceebc5c9e2e5f4 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -313,6 +313,7 @@ Benjamin Collar
 Jeffery Collins
 Robert Collins
 Paul Colomiets
+Samuel Colvin
 Christophe Combelles
 Geremy Condra
 Denver Coneybeare
diff --git a/Misc/NEWS.d/next/Library/2019-01-20-12-37-52.bpo-35788.Kj6705.rst b/Misc/NEWS.d/next/Library/2019-01-20-12-37-52.bpo-35788.Kj6705.rst
new file mode 100644
index 00000000000000..e390a408f1c8fb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-01-20-12-37-52.bpo-35788.Kj6705.rst
@@ -0,0 +1 @@
+Fix regressions in ``smtpd.PureProxy`` and ``smtpd.MailmanProxy``.