updated_logic.ex

  def call(%{email: email}) do
    with :ok <- validate_email(email) do
      email
      |> build_changeset
      |> create_user
    end
  end

  #...

  defp validate_email(email) do
    case String.match?(email, ~r/@/) do
      true -> :ok
      false -> {:error, :invalid_email}
    end
  end