@type t() :: %Plover.Response.Envelope{
  bcc: [Plover.Response.Address.t()],
  cc: [Plover.Response.Address.t()],
  date: nil | String.t(),
  from: [Plover.Response.Address.t()],
  in_reply_to: nil | String.t(),
  message_id: nil | String.t(),
  reply_to: [Plover.Response.Address.t()],
  sender: [Plover.Response.Address.t()],
  subject: nil | String.t(),
  to: [Plover.Response.Address.t()]
}