This is a message for the man which I will be in five years. But, before going on, I have to think about identification.
The question is whether we can say the person which I will be in five years is still me. For example, Locke maintained that the basis of identification is memory. According to him, me in the past and today's me are the same person if and as far as I can remember the past self. Therefore, if I cannot remember leaving this message five years from now, the man who wrote it is different from the me in the future. And I am sure I won't remember it. Then, what is the point of this message?
In my opinion, it is still meaningful in spite of the doubt about the identity, for what can be more wonderful than receiving a letter written five years ago? So I conclude failure to ascertain the identity between the two persons is no problem.
If you are reading this entry in 2021, and your name is ai_no_coffee_rumba, could you just evaluate the argument I presented above? And if you like, you can do me a favor and build your own argument about the doubt in identity and the meaningfulness of sending a message to oneself in the future. You can, no, you must be able to develop a more interesting and convincing argument.