anchor.diff:  |metadata|JSON object|Transmission level metadata containing key/value pairs |no| Metadata is available during events through the Webhooks and is provided to the substitution engine.  A maximum of 200 bytes of merged metadata (transmission level + recipient level) is available with recipient metadata taking precedence over transmission metadata when there are conflicts.  |
   |substitution_data|JSON object|Key/value pairs that are provided to the substitution engine| no | Recipient substitution data takes precedence over transmission substitution data. Unlike metadata, substitution data is not included in Webhook events. |
   |return_path | string |Email to use for envelope FROM ( **Note:** SparkPost Elite only )| yes | To support Variable Envelope Return Path (VERP), this field can also optionally be specified inside of the address object of a specific recipient in order to give the recipient a unique envelope MAIL FROM. |
  -|content| JSON object | Content that will be used to construct a message | yes | Specify a stored template or specify inline template content. When using a stored template, specify the "template_id" as described in Using a Stored Template.  Otherwise, provide the inline content using the fields described in the Templates API documentation for Content Attributes.  Maximum size - 15MBs|
  +|content| JSON object | Content that will be used to construct a message | yes | Specify a stored template or specify inline template content. When using a stored template, specify the "template_id" as described in Using a Stored Template.  Otherwise, provide the inline content using the fields described in Inline Content Attributes.  Maximum size - 10MBs|
   |total_recipients | number | Computed total recipients | no | Read only|
   |num_generated | number | Computed total number of messages generated | no |Read only|
   |num_failed_generation| number| Computed total number of failed messages | no | Read only|
  
positive.code: * Each header value is expected in the UTF-8 charset without RFC2047 encoding.
  * Substitution syntax is supported.
  
  #### email_rfc822 Notes
  
  Alternately, the content JSON object may contain a single "email_rfc822" field.  email_rfc822 is mutually exclusive with all of the above fields.
  
  | Field         | Type     | Description                           | Required   | Notes   |
  |--------------------|:-:       |---------------------------------------|-----------------------|--------|
  |email_rfc822    |string  |Pre-built message with the format as described by the [message/rfc822 Content-Type](http://tools.ietf.org/html/rfc2046#section-5.2.1) |no   |  See the email_rfc822 Notes. |
  
  * Substitutions will be applied in the top-level headers and the first non-attachment text/plain and
  first non-attachment text/html MIME parts only.
  
negative.code:           }
          }
  
  + Request Create Transmission with inline images (application/json)
  
      + Headers
  
              Authorization: 14ac5499cfdd2bb2859e4476d2e5b1d2bad079bf
  
      + Body
  
          ```
          {
            "campaign_id" : "inline_image_example_sparkpost_elite_only",
            "recipients": [
              {
                "address": "wilma@flintstone.com"
              }
            ],
            "content": {
              "from": {
                "email": "marketing@company.example",
                "name": "Example Company"
              },
  
              "subject": "Inline image example",
              "html": "<html><body>Here is your inline image!<br> <img src=\"cid:my_image.jpeg\"></body></html>",
              "images" : [
                {
                  "type" : "image/jpeg",
                  "name" : "my_image.jpeg",
                  "content" : "VGhpcyBkb2Vzbid0IGxvb2sgbGlrZSBhIGpwZWcgdG8gbWUh"
                }
              ]
            }
          }
          ```
  
  + Response 200 (application/json)
  
      + Body
  
          ```
          {
            "results": {
              "total_rejected_recipients": 0,
              "total_accepted_recipients": 1,
              "id": "11668787484950529"
            }
          }
  
  
  
  ## Retrieve [/transmissions/{id}]
  