def mail_send(from:, from_name:, to:, subject:, body:)
      mail = build_mail(from, from_name, to, subject, body)
      res = client.mail._('send').post(request_body: mail.to_json)
      @last_response = Response.new(res)
    end
def mail_send(from:, from_name:, to:, subject:, body:)
      mail = build_mail(from, from_name, to, subject, body)
      res = client.mail._('send').post(request_body: mail.to_json)
      @last_response = Response.new(res)
    end
TODO: status_code が 500 系およびタイムアウトのときにリトライできるようにする
 POST /mail/send