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1) Agreed completely.

2) smtpd.py is written using the asynchat/asyncore
framework.  This uses the single/multi-threaded/forked
SocketServer.TCPServer framework (various protocols have
both an asynchronous and synchronous version in the standard
library).

Other comments:
3) I also think documentation for this module is necessary,
as is a 3rd party review of the code and error messages.

4) SSL/TLS support should also be included, along with a
reasonable assortment of EHLO, HELP, etc., though smtpd also
should gain such support, so this is not a deal-breaker.

5) Perhaps smtpd should gain allow_sender() and
allow_recipient() methods, and both should gain allow_host()*.

*proper implementations of the above 3
allow_(sender|recipient|host) methods (with either
framework) would be sufficient to do temporary hostname
blacklisting on remote hosts trying to brute-force a local
account listing.