How can I make my microwave oven door fit perfectly?

Recently I have bought a new microwave (whirlpool VT 255) and I noticed that the door doesn't fit perfectly. When I close it, there is still a 3 mm gap on the top between the door and the oven. I'm not able to see into the microwave through the gap but I'm wondering if there is a possible risk of microwave leakage. Is there a way how to make the door fit properly? I have heard that microwaves don't leak through openings smaller than their wavelength. Does it mean they certainly wont leak out of such a small gap?