General Notes
--------------
 * MX is NOT required, but an A record, or CNAME to a MX MUST be present at the least.
 * EHLO should be tried, then fall back to HELO
 * The 250 return code from RCPT TO is not actually clear-cut. A 251 may be
   returned if the message was forwarded to another address.  This could be a
   useful indicator to end-users that an address should be updated.
 * RCPT TO can accpet just "postmaster" without a domain name
 * Server MUST not close connection before:
   - QUIT and returning 221 response
   - Forced requirement, and only after returning a 421 response
   - Clients expriencing a forced connection closure, without prior warning should
     just treat it like a 451 closure regardless
 * ALWAYS use blocking sockets for the initial connection (this should prevent
   Exim4's whining about sync).

Response codes
--------------
 * From RFC2821, 4.2.
   - In particular, the 220, 221, 251, 421, and 551 reply codes
     are associated with message text that must be parsed and interpreted
     by machines.

Error Codes
------------
 * Numeric 5yz = Error
   - 550/RCPT TO = Relay denied
   - 500 = Unknown command
 * Numeric 2yz = Normal
 * Numeric 4yz = Temporary failure??

<?php

class EsmtpTransport implements Transport, EsmtpCommandWriter {
}

interface EsmtpCommandWriter {
  public function getBuffer();
  public function executeCommand($command, $expectedCodes);
}

interface Extension {
  public function getKeyword();
  public function afterEhlo($commandWriter);
  public function getRcptParams();
  public function getMailParams();
  public function atCommand($commandWriter, $command, $expectedResponse) throws CommandSentException;
}