@type t() :: %Inttegro.MessageTemplateEmailContent{
  from_: Inttegro.MessageTemplateMailbox.t() | nil,
  headers: %{optional(String.t()) => String.t()} | nil,
  html: String.t(),
  reply_to: Inttegro.MessageTemplateMailbox.t() | nil,
  subject: String.t()
}