diff -r c43362d35d8d Lib/smtpd.py
--- a/Lib/smtpd.py	Fri Jul 04 17:00:25 2014 -0700
+++ b/Lib/smtpd.py	Wed Jul 16 21:53:15 2014 +0200
@@ -303,7 +303,15 @@
         elif limit:
             self.num_bytes += len(data)
         if self._decode_data:
-            self.received_lines.append(str(data, 'utf-8'))
+            try:
+                self.received_lines.append(str(data, 'utf-8'))
+            except UnicodeDecodeError:
+                self.push('451 Internal server error: Unsuccessfully tried to'
+                          ' decode input. Decoding should be disabled by the'
+                          ' administrator (possible in Python 3.5 or later, '
+                          ' see'
+                          ' https://docs.python.org/3.5/library/smtpd.html)')
+                self.smtp_state = self.COMMAND
         else:
             self.received_lines.append(data)
 
diff -r c43362d35d8d Lib/test/test_smtpd.py
--- a/Lib/test/test_smtpd.py	Fri Jul 04 17:00:25 2014 -0700
+++ b/Lib/test/test_smtpd.py	Wed Jul 16 21:53:15 2014 +0200
@@ -647,6 +647,20 @@
             b'utf8 enriched text: \xc5\xbc\xc5\xba\xc4\x87\n'
                 b'and some plain ascii')
 
+    def test_non_utf8_8bit_data(self):
+        # test for issue 19806.
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL From:eggs@example')
+        self.write_line(b'RCPT To:spam@example')
+        self.write_line(b'DATA')
+        self.write_line(b'undecodable bytes: \xc5\x00\xc4\xff\xd0')
+        self.write_line(b'and some plain ascii')
+        self.write_line(b'.')
+        self.assertEqual(
+            self.channel.received_data,
+            b'undecodable bytes: \xc5\x00\xc4\xff\xd0\n'
+            b'and some plain ascii')
+
 
 class SMTPDChannelWithDecodeDataTrue(unittest.TestCase):
 
@@ -691,6 +705,24 @@
             self.channel.received_data,
             'utf8 enriched text: żźć\nand some plain ascii')
 
+    def test_non_utf8_8bit_data(self):
+        # test for issue 19806.
+        self.write_line(b'HELO example')
+        self.write_line(b'MAIL From:eggs@example')
+        self.write_line(b'RCPT To:spam@example')
+        self.write_line(b'DATA')
+        self.write_line(b'undecodable bytes: \xc5\x00\xc4\xff\xd0\n')
+        self.assertEqual(
+            self.channel.socket.last,
+            b'451 Internal server error: Unsuccessfully tried to'
+            b' decode input. Decoding should be disabled by the'
+            b' administrator (possible in Python 3.5 or later, '
+            b' see'
+            b' https://docs.python.org/3.5/library/smtpd.html)\r\n'
+        )
+        self.assertEqual(self.channel.received_data, '')
+        self.assertEqual(self.channel.smtp_state, self.channel.COMMAND)
+
 
 if __name__ == "__main__":
     unittest.main()