* Newsletters::ValidateNewsletter
 *
 *  Validate the given newsletter details.
 *
 *  newsletterid [required* - int] -- selected newsletter
 *  newslettername [required* - string] -- selected newsletter
 *      *Exactly one of either newsletterid or newslettername is required
 *
 *  sendfromemail [optional - string] -- send-from email to be used when sent
 *  sendmultipart [optional - string] -- send as multipart, default true
 */
{
  "username": "bob",
  "usertoken": "abc",
  "requesttype": "Newsletters",
  "requestmethod": "ValidateNewsletter",
  "details": {
    "newsletterid": "{*newsletter_id*}", /* required if no newslettername */
    "newslettername": "{*newsletter_name*}", /* required if no newsletterid */
    "sendfromemail": "{*send_from_email*}",
    "sendmultipart": "{*bool*}"
  }
}