I think a small message like Hello can change quite a bit,
say you are about to do something, but you receive an email saying Hello from yourself, you usually get distracted enough by mails to at least read them, and something like that would really surprise me, it's a message from me, but I don't remember sending it, so I start thinking about it ending up not doing what I was supposed to do, or doing it much later when I decide not to bother myself about the mail itself, in my case that wouldn't happen, I would definitely stop doing what I was supposed to do and start thinking what is the meaning of this, that would change the fact I was meant to do something into I didn't do it, and that change could very well destroy the world, depending on what I was supposed to do and that event could lead to infinite numbers of events.
Possibilities of interfering in history are infinite and consequences of simple messages can be harmful in so many ways you might not even think of.