{
    "From": {
        "Address": "code@exacttarget.com",
        "Name": "EmailTest"
    },
    "To": {
        "Address": "recipient@email.com",
        "SubscriberKey": "recipient@email.com",
        "ContactAttributes": {
            "SubscriberAttributes": {
                "content" : "hello"
            }
        }
    }
}

$ curl -H 'Authorization: Bearer SessionId' -H 'X-PrettyPrint:1' -H 'Content-Type: application/json' -d @jsonfile BaseURL/services/data/v36.0/actions/standard/emailSimple

{
  "inputs" : [
    {
      "emailBody" : "This is the body of the email",
      "emailAddresses" : "user1@email.com,user2@email.com",
      "emailSubject" : "An email from salesforce",
      "senderType" : "CurrentUser"
    }
  ]
}