Use backer rod (closed cell polyethylene foam, also called filler rope) to fill any wide gaps. It comes in diameters from 1/4" to 1 1/2". Since you have a steel door, don't use caulk, use silicone sealer. You can use clear but you can also buy it in several different colors to match the color of the door or trim. Apply it to the perimeter of the outside of the door.
The beauty of sealing the door this way is... It is cheap, it is easy, and it can be undone at a later date.