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 that will be hidden from all other recipients | no | A unique message will be generated for each recipient in this list. |
  -| archive | JSON array | Array of recipient addresses that will be hidden from all other recipients | no | A unique message will be generated for each recipient in this list. |
  +| cc | JSON array | Array of recipient addresses that will be included in the Cc header (SparkPost.com only) | no | A unique message will be generated for each recipient in this list. |
  +| bcc | JSON array | Array of recipient addresses that will be hidden from all other recipients (SparkPost.com only) | no | A unique message will be generated for each recipient in this list. |
  +| archive | JSON array | Array of recipient addresses that will be hidden from all other recipients (SparkPost.com only) | no | A unique message will be generated for each recipient in this list. For a full description, see the section "**What is an archive recipient?**" |
   | 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:     }
      ```
  
  ## 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,
  
negative.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.|
  
  ## Sending Messages with cc, bcc, and archive Recipients
  
  When submitting an email via SMTP that includes the X-MSYS-API header, you may specify a JSON array for cc, bcc, and archive lists.  For each address in each of these arrays, a message will be generated. Messages will be generated with the following headers: 
  * It is the responsibility of the user to include their own "To" header in the body of the message.
  * All messages will display the "Cc" header and the value of that header will include all addresses listed in the "cc" array.
  * The "bcc" recipients will each receive a message with the "To" and "Cc" headers described above and, additionally, will see a "Bcc" header with ONLY their own recipient address as the value of the header.
  * The "archive" recipients will each receive a message with the "To" and "Cc" headers described above however, they will not have a "Bcc" header.
  
  
  **What is an archive recipient?**
  
  Recipients in the "archive" list will receive an exact replica of the message that was sent to the RCPT TO address. In particular, any encoded links intended for the RCPT TO recipient will appear "as is" in the archive messages.  In contrast, recipients in the “bcc” list will have links encoded specific to their address. (Note: There will be some differences in headers such as X-MSFBL or List-Unsubscribe headers.)
  
  For example:
  
  ```
  X-MSYS-API: {
  
     "cc" : [ "cc_email1@corp.com", "cc_email2@corp.com" ], 
     "bcc" : [ "bcc_email1@corp.com", "bcc_email2@corp.com" ], 
     "archive" : [ "archive_email@corp.com" ], 
  
     "options" : {"open_tracking" : false, "click_tracking" : true},
  }
  ```
  NOTE:  You may not specify more than a total of 1000 total recipients in those 3 lists.
  