Queue one explicitly requested plain-text email

curl -X POST "https://example.com/v1/mail/send" \
  -H "Content-Type: application/json" \
  -d '{
    "operationId": "3051932a-fdd2-48fa-b330-7e7d41535969",
    "to": "user@example.com",
    "subject": "string",
    "text": "string"
  }'

{
  "operationId": "3051932a-fdd2-48fa-b330-7e7d41535969",
  "mailboxId": "f16f762c-c61b-441d-8b9c-9aff4650cbe1",
  "state": "queued",
  "contentRetained": true,
  "recipientCount": 1,
  "receipt": {
    "available": true,
    "createdAt": "2019-08-24T14:15:22Z",
    "firstClaimAt": "2019-08-24T14:15:22Z",
    "acceptanceObservedAt": "2019-08-24T14:15:22Z",
    "suppressedAt": "2019-08-24T14:15:22Z",
    "stoppedAt": "2019-08-24T14:15:22Z",
    "submissions": 0
  }
}