anchor.diff:  |-------|------|-------------|----------|-------|
   | campaign_id | string | Name of the campaign to associate with the SMTP message | no | Maximum length - 64 bytes (same restriction as the REST API) |
   | metadata | JSON object | JSON key value pairs associated with the SMTP message | no | A maximum of 200 bytes of metadata is available during click/open events. |
  +| cc | JSON array | Array of recipient addresses that will be included in the CC header | no | A unique message will be generated for each recipient in this list. |
  +| bcc | JSON array | Array of recipient addresses. Will not be included in any header | no | A unique message will be generated for each recipient in this list. |
  +| archive | JSON array | Array of recipient addresses. Will not be included in any header | no | A unique message will be generated for each recipient in this list. |
   | tags | JSON array | Array of text labels associated with the SMTP message | no | Tags are available in click/open events. Maximum number of tags is 10 per recipient, 100 system wide. |
   | options | JSON object | JSON object in which SMTP API options are defined | no | For a full description, see the Options Attributes. |
   
  
positive.code: | click_tracking | boolean | Whether click tracking is enabled for this SMTP message | no | Defaults to false. |
  | sandbox|boolean|Whether or not to use the sandbox sending domain | no |Defaults to false.|
  
  SMTP defines a maximum line length of 1000 characters (including CRLF).  If the value of the X-MSYS-API JSON string is
  longer than 998 characters, it will need to be folded across multiple lines before the message is injected.  An example
  of a folded header:
  
negative.code:     }
      ```
  
  ## Group SMTP API
  
  Through use of the X-MSYS-API header in a message sent to SparkPost through SMTP 
  (see ["Sending an Email"](https://www.sparkpost.com/docs/sending-email)), you can now specify a campaign id, metadata,
  