Avoiding the Paradox
Quite simply, the laws of the universe allow for theoretical travel to the future (by travelling faster than the speed of light time passes "slower" for you than for the outside world), but not to the past.
If you found a way to send something to the past, you would most likely actually be sending it to a similar, but parallel universe at a point in "time" equivalent to your universe's past. Even if a parallel universe had not previously existed, the very fact that you meddled with space time would cause that parallel universe to come into existence.
What that means is that the parallel you would be warned of a very likely negative event in the future, and would - we assume - take action to avoid it. I say a likely negative event because as similar as those parallel universes would be you cannot guarantee that your interference will not set other events off which will result in other, worse, or better outcomes.
Thus, his future would be different. But not yours.
This is why no loophole is created.
The Show Must Go On
However, if your present self isn't reaping the benefit of this effort why bother sending a message in the first place? What's his motivation?
This is where you, as the author, get to have some fun.
Let us assume that the second that message is send to the past a similar, but parallel universe is created - instantiated at that point in time at which the note was "aimed" at.
That universe had potentially existed all along, however your act of sending an item there has caused it to "come into focus", shall we say.
You now shift the story from Universe #1 to Universe #2. You make a note that the original character (Protagonist #1) sent a number of messages to the "past", not realizing that each time he was doing so he was "instantiating" a new parallel universe, and wreaking havoc in space time continuum. He never saw any results, and eventually gave up, assuming that his technology doesn't work, and not realizing its effects. He had basically spammed several parallel universes with various consequences.
However, you choose to focus on Protagonist #2. He gets the message, figures out its significance, and maybe even makes a note that he should send a message to himself in the past in order to avoid the incident altogether again - making the same flaw in his assumptions as Protagonist #1, and also ending up spamming the multiverse.
Your story can then continue in Universe #2.