2. Refactor to use more descriptive names.
3. Enhance tests in main().

now idempotent.

See issue 1324.

in response that keeped having a non-ascii character.

Neal: please backport!

When "MAIL" is received without args, an exception happens instead of
sending a 501 syntax error response.

while another thread uses it.

  configure and setup python on different platforms -- at least
  in theory.)
* Move the Python on Mac docs in that chapter.
* Add a new chapter about the command line invocation, by stargaming.