       iex> email = new |> from("tony@stark.com")
       %Swoosh.Email{from: {"", "tony@stark.com"}, [...]}
       iex> Memory.push(email)
-      %Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "A1B2C3"}, [...]}
+      %Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "a1b2c3"}, [...]}
   """
   def push(email) do
     GenServer.call(__MODULE__, {:push, email})

      iex> email = new |> from("tony@stark.com")
      %Swoosh.Email{from: {"", "tony@stark.com"}, [...]}
      iex> Memory.push(email)
      %Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "A1B2C3"}, [...]}
      iex> Memory.all()
      [%Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "A1B2C3"}, [...]}]
  """
  def all() do
    GenServer.call(__MODULE__, :all)

      iex> email = new |> from("tony@stark.com")
      %Swoosh.Email{from: {"", "tony@stark.com"}, [...]}
      iex> Memory.push(email)
      %Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "a1b2c3"}, [...]}
      iex> Memory.all() |> Enum.count()
      1
      iex> Memory.pop()
      %Swoosh.Email{from: {"", "tony@stark.com"}, headers: %{"Message-ID": "a1b2c3"}, [...]}
      iex> Memory.all() |> Enun.count()
      0
  """
