The main content of the Send API payload will be the Messages property, which is a collection of messages, represented as a JSON array. This is the only mandatory property within the payload.
Additionally, you can use the Globals property to specify values for properties to be applied to all objects within the Messages array.
Finally, you can enable Sandbox mode by setting the SandboxMode property to true.
Full descriptions of the properties of each message can be found in the Email API reference