Hi Barry, I was testing this and it seems to work, am I doing something wrong in order to reproduce it? I've used the same parameters from the unit tests

Python 3.5.0+ (3.5:1e99ba6b7c98, Oct  8 2015, 17:12:06) 
[GCC 4.8.4] on linux
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>>> import smtpd
>>> smtpd.SMTPServer(("127.0.0.1", 0), ('b',0),enable_SMTPUTF8=True,decode_data=True)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/maurosr/dev/cpython/Lib/smtpd.py", line 645, in __init__
    raise ValueError("The decode_data and enable_SMTPUTF8"
ValueError: The decode_data and enable_SMTPUTF8 parameters cannot be set to True at the same time.