I'm not sure what the movie name was, but it was in theaters somewhere between 2003-2005, or so I think (may be wrong :(...).
Anyway, the movie is about these two people who write letters to each other. The girl lived 2 years ago and the guy is in the present. I remember a tree house... Sorry for the lack of detail, I watched the trailer like 3 years ago so I don't really remember much.
Thanks!
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