diff -r 3291e6132a67 Lib/smtpd.py
--- a/Lib/smtpd.py	Wed Oct 07 11:15:15 2015 +0000
+++ b/Lib/smtpd.py	Fri Oct 09 14:21:16 2015 +0800
@@ -137,8 +137,8 @@
         self.enable_SMTPUTF8 = enable_SMTPUTF8
         if enable_SMTPUTF8:
             if decode_data:
-                ValueError("decode_data and enable_SMTPUTF8 cannot be set to"
-                           " True at the same time")
+                raise ValueError("decode_data and enable_SMTPUTF8 cannot"
+                                 " be set to True at the same time")
             decode_data = False
         if decode_data is None:
             warn("The decode_data default of True will change to False in 3.6;"
diff -r 3291e6132a67 Lib/test/test_smtpd.py
--- a/Lib/test/test_smtpd.py	Wed Oct 07 11:15:15 2015 +0000
+++ b/Lib/test/test_smtpd.py	Fri Oct 09 14:21:16 2015 +0800
@@ -313,6 +313,12 @@
             DummyDispatcherBroken, BrokenDummyServer,
             (support.HOST, 0), ('b', 0), decode_data=True)
 
+    def test_decode_data_and_enable_SMTPUTF8_raises(self):
+        self.assertRaises(
+            ValueError, smtpd.SMTPChannel,
+            self.server, self.channel.conn, self.channel.addr,
+            enable_SMTPUTF8=True, decode_data=True)
+
     def test_server_accept(self):
         self.server.handle_accept()