Il Mare (2000) on Netflix

Plot summary

Eun-joo is leaving Il Mare, a house beside the sea, and before she goes she puts a Christmas card in its mailbox for whoever moves in next, with a request that her letters be sent on to her. Sung-hyun, an architecture student, finds the card and cannot make sense of it: he is the first person to live in the house, and the date on the card is two years ahead of him. He writes back to say so. Letters keep crossing between them, and the two work out that they are living two years apart and that the mailbox is what lets them reach each other. Eun-joo asks Sung-hyun to recover a tape player she lost, and he finds it.