How can I stop my doors from leaking at each corner?
Usually†, an external door has a weatherboard with a drip-channel and a sealing system. I'd check that this is intact. On a badly fitted door, the door can sag and cause the flexible sealing strips to be torn off.



There's usually also a sealing strip in the frame at sides and top, but this is mostly for draughtproofing.



If water is pooling outside during heavy rain, I'd install better drainage. Normal doors are not an effective flood barrier.



† in the UK. Doors differ elsewhere.