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