"the lake house" movie

So me and my boyfriend watched it together about 3 day's ago, it's a wonderful movie! I really enjoyed it (: I just dont understand how they live 2 year's apart and are writing back in forth thru letters, is the mailbox like a time portle or something! I got very confused... please help me out it would be appreciated!

They're not actually living two years apart, I mean they do live at the same time, just in different places... they're simply communicating two years apart... (which is why it takes two years before they can meet) and yes the mailbox is kinda like a time portal transporting the letter back between the present and the past