curl --request POST \
  --url https://api-v2.missioninbox.com/relay_servers/ \
  --header 'Content-Type: application/json' \
  --data '{
  "name": "<string>",
  "transporter_provider": "<string>",
  "transporter_key": "<string>"
}'

{
  "id": 123,
  "domain_count": "<string>",
  "mailbox_count": "<string>",
  "name": "<string>",
  "transporter_key": "<string>",
  "transporter_provider": "<string>",
  "slug": "<string>",
  "users": [
    123
  ]
}