diff -r ca696ca204e0 Lib/smtpd.py
--- a/Lib/smtpd.py	Sat Aug 09 16:40:49 2014 -0400
+++ b/Lib/smtpd.py	Tue Aug 12 11:51:04 2014 +0200
@@ -122,10 +122,8 @@
 
     @property
     def max_command_size_limit(self):
-        try:
-            return max(self.command_size_limits.values())
-        except ValueError:
-            return self.command_size_limit
+        return max(self.command_size_limits.values(),
+                   default=self.command_size_limit)
 
     def __init__(self, server, conn, addr, data_size_limit=DATA_SIZE_DEFAULT,
                  map=None, enable_SMTPUTF8=False, decode_data=None):
@@ -137,8 +135,8 @@
         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;"
@@ -747,7 +745,7 @@
 
 class PureProxy(SMTPServer):
     def __init__(self, *args, **kwargs):
-        if 'enable_SMTPUTF8' in kwargs and kwargs['enable_SMTPUTF8']:
+        if kwargs.get('enable_SMTPUTF8'):
             raise ValueError("PureProxy does not support SMTPUTF8.")
         super(PureProxy, self).__init__(*args, **kwargs)
 
diff -r ca696ca204e0 Lib/test/test_smtpd.py
--- a/Lib/test/test_smtpd.py	Sat Aug 09 16:40:49 2014 -0400
+++ b/Lib/test/test_smtpd.py	Tue Aug 12 11:51:04 2014 +0200
@@ -176,6 +176,15 @@
              ------------ END MESSAGE ------------
              """))
 
+    def test_decode_data_and_enable_SMTPUTF8_raises(self):
+        server = smtpd.DebuggingServer(
+            (support.HOST, 0), ('b', 0), decode_data=True)
+        conn, addr = server.accept()
+        self.assertRaises(
+            ValueError,
+            smtpd.SMTPChannel,
+            server, conn, addr, enable_SMTPUTF8=True, decode_data=True)
+
     def tearDown(self):
         asyncore.close_all()
         asyncore.socket = smtpd.socket = socket