I'm now checking it in.  I need to write some documentation for it,
but I don't have time right now.  Still, I wanted to get this into
2.1a2.

# Overview:
#
# This file implements the minimal SMTP protocol as defined in RFC 821.  It
# has a hierarchy of classes which implement the backend functionality for the
# smtpd.  A number of classes are provided:
#
#   SMTPServer - the base class for the backend.  Raises an UnimplementedError
#   if you try to use it.
#
#   DebuggingServer - simply prints each message it receives on stdout.
#
#   PureProxy - Proxies all messages to a real smtpd which does final
#   delivery.  One known problem with this class is that it doesn't handle
#   SMTP errors from the backend server at all.  This should be fixed
#   (contributions are welcome!).
#
#   MailmanProxy - An experimental hack to work with GNU Mailman
#   <www.list.org>.  Using this server as your real incoming smtpd, your
#   mailhost will automatically recognize and accept mail destined to Mailman
#   lists when those lists are created.  Every message not destined for a list
#   gets forwarded to a real backend smtpd, as with PureProxy.  Again, errors
#   are not handled correctly yet.