TODO

- DKIM (need a node dkim library first - perhaps based on libdkimpp?)
- Rate Limiting for outbound mail
- Milter support
- Ability to modify the body of email (e.g add a banner) and have it re-write the data_lines
- Create a config file for each of the core shipping configs, so people have something as a baseline
- IMAP server (long shot for now)
- Add the option to define special/important plugins that are always run, even if an earlier 
  plugin on the same hook returns OK
- Spool data to disk once databytes reaches a certain size to save memory
  - The plan for this is as follows:
    1. Create a subclass of Stream which is read/writable, and spools to
       memory at first, and then to disk after a configurable size is
       reached.
    2. Convert code that reads data_lines to use that class instead
    3. Convert code that sends over a socket to use .pipe() instead - see
       how outbound.js does that as it is reading from disk.
    4. Somehow work into all that ability to re-write the email after
       adding banners etc.

Outbound improvements
 - Fix --qlist and --qstat (currently broken)
 - Add the ability to force a run on a specific queue file or destination domain
 - Make retry times configurable (handle RFC requirements for 5 days and DSN queued warnings)
 - Limit concurrency by domain
 - Disable deliveries for a domain
 - Pool connections by domain/MX