* ADD a new class, `test_email_server.SMTPTestsMixin`, which has the
   following methods useful for testing SMTP within
   `t.t.unittest.TestCase` subclasses:

     - SMTPTestsMixin._buildEmail(fromAddr=None, toAddr=None,
                                  subject=None, body=None)
         Creates email text (including headers) for use in an SMTP DATA
         segment. Includes the SMTP DATA EOM command ('.') at the
         end. If no keyword arguments are given, the defaults are fairly
         sane.

     - SMTPTestsMixin._buildSMTP(commands)
         Format a list of SMTP protocol commands into a string, using
         the proper protocol delimiter.

     - SMTPTestsMixin._test(commands, expected)
         Send the commands to the TestCase's protocol (this must be the
         `proto` attribute of your `TestCase`), and then check that the
         expected output matches what was received from
         `TestCase.transport` (another attribute you must assign).