@callback deliver_async(email :: email()) :: {:ok, term()} | {:error, term()}

@callback deliver_inline(email :: email()) :: {:ok, term()} | {:error, term()}

@callback from(email(), String.t() | {String.t(), String.t()}) :: email()

@callback html_body(email(), String.t()) :: email()

@callback new() :: email()

@callback new(term()) :: email()

@callback subject(email(), String.t()) :: email()

@callback text_body(email(), String.t()) :: email()

@callback to(email(), String.t() | {String.t(), String.t()}) :: email()