Jamsill Guard Door & Window Sill Pan Flashing
All most no one ever takes the time to install a door correctly.
If someone pored a stoop or built a deck or stoop right up tight to the doors threshold water will always be getting in there. There needed to be a min. of a 4" step down, 7" is far better.
At least a dozen times a year we have to remove doors that this was done on and most often end up replaing the whole inside floor and sometimes even the rim joist, floor joist, in one case it had gone on for 10 years and even the foundtaion plate was rotted out.
If they did not wrap the house wrap around the roughed in framing, install a sill pan for the door to sit in, install 6" wide window and door tape around the opening before installing the door.
Once the door was sat in place and nailed, a piece of Z molding needed to be installed over the top brick moulding, and a piece of 3/4 X 4 Or 6" vinyl lumber needed to be pushed up againt the bottom of the threshold and be screwed into place with stainless steel or ceramic coated trim head screw to fully support the threshold. Try stepping on your threshold in the middle and see if it does not flex causing the two sides near the jams to move and crack the sealing area.
If there was a sill pan there it would not matter because the water would be forsed outside.
Once all that's done the J moulding can go on starting at the bottom, the two sides then the top piece. The top piece must have a tab cut and bent over so water will run past the seams on the sides.
If a siding job is done correctly there should never be a need for caulking. Caulking will crack at some point and leak.
If someone does take the time to remove the door have them take the time to remove the wooden brick moulding and replace it with vinyl so it will never rot.