Add the eggs, mustard, vinegar and salt to a food processor or blender. Process for 30 seconds in the food processor, or for 10 seconds in the blender. With the motor running, drizzle in the oil slowly at first, then add in a thin, steady steam until all the oil is added and the mixture is smooth. Stop the motor and taste. If the sauce is too thick, thin it with a little hot water. If too thin, process a little longer. The mayonnaise can be refrigerated in an airtight container for up to 1 week.