there are a number of things that will help with this the first is a foam strip that you can buy from Leroy Merlin for around €8
Sometimes the problems is the level of the approach to the door, so just put a level over it to see which way the water runs and you can add a threshold step on the outside of the door, to help prevent this, a simple step to build, or piece of treated timber
The last solution will sound slightly odd, we find that sometimes the problems is that the water hits and runs down the front door, pooling at the bottom and entering the property and there are 2 solution that can prevent this the first is a simple porch roof cover, again from Leory Merlin, which stops the rain dropping onto the door and front area
What the Spanish do is put a simple roller blind over the front door, again stopping the water hitting the door and rolling down
We have used all of the above solutions on various properties but any barrier ideally needs to be on the outside of the door