Here we have a movie without a time machine unless you consider the magic mailbox to be a machine. This romantic little picture starring Sandra Bullock reminded me of "Somewhere in Time", that sad movie starring Christopher Reeves AKA SUPERMAN. If only he could have seen the future, that Superman would spend his life in a wheelchair.
Getting back to the movie "Lake House". It deals with a guy and a gal who meet and then are separated. The method of communication is a magical mailbox, much like in the movie "Frequency" we have a very special shortwave radio.
Interesting that in the Movie Lake House the male romantic lead is Keanu Reeves, who has matured a lot since "Bill & Ted's Excellent Adventure" a quite different time travel movie. In that one a magic phone booth acted as a time machine.
Still, the question is "Is the movie Lake House about time travel?" I believe the answer is YES!