Shows how much different is the direct message send and `erlang:send_after(0, self(), ...)`.

send_after_test.erl

-module(send_after_test).

-export([main/0]).

main() ->
    Self = self(),
    Ref = make_ref(),
    spawn(fun() -> test(Self, Ref) end),
    receive Ref -> ok end.

test(Master, Ref) ->
    Self = self(),

    Self ! 0,
    erlang:send_after(0, Self, send_after),
    send(Self, 1, 1000),

    recv(0),
    Master ! Ref.

send(Self, I, Max) ->
    Self ! I,
    if I >= Max ->
            ok;
       true ->
            send(Self, I + 1, Max)
    end.

recv(N) ->
    receive
        send_after ->
            io:format("Received after ~p messages\n", [N]);
        N ->
            recv(N + 1);
        Msg ->
            io:format("Unexpected message ~p\n", [Msg]),
            recv(N + 1)
    end.