Because they're heavier than hollow doors, exterior doors tend to sag over time due to age, faulty installation or stripped screws. The problem almost always originates with the hinges. If the top hinge loosens only slightly, the door will sag forward, catching or rubbing on the doorjamb. If it's bad enough, you can even start seeing daylight above the door. You don't have to live with it. Get out your tools and repair it in a few minutes using a few screws long enough to penetrate where the old screws lost their grip.
Open the door to 90 degrees. Place a shim under the outside corner if needed to level the door horizontally. Use a drill/driver to remove the screws from the top hinge on the door.
Replace the screws with 2-inch screws or screws that are at least 3/4-inch longer than the original screws. Drive them into the original screw holes using a drill/driver.
Remove the screws from the jamb side of the door. Insert 3-inch screws in the holes.
Drive the screws in equally halfway. The screws will penetrate all the way through the jamb and into the stud framing behind the jamb.
Drive the top screw in tight using force if necessary. As the screw bites into the stud, the jamb may move back slightly; that's fine and to be expected. Finish by driving the other two screws in tight.