In Experimental, there are two catchment areas: a wide catchment area for passengers (who can walk long distances to a stop) and a narrow catchment area for freight (which has no legs and thus cannot walk). Mail works on the passenger catchment area on the basis that people can carry letters as far as they can walk for posting. By default, the smaller catchment area is shown for goods only stops; the larger catchment area for other stops, but the converse can be shown by holding down SHIFT when selecting the stops in the menu.
However, as Rollermaterial points out, there is more to spacing stops than just ensuring that everything is covered by a catchment area. In Experimental, as in real life, passengers will prefer to travel from nearer than farther stops. Each passenger has a journey time budget, and will (1) take the quickest journey to its destination; and (2) not travel at all if the travel time exceeds the budget. The time taken walking from the origin building to the first stop (at 4km/h, assuming walking in a straight line) is counted as part of the journey time. Thus, it is not enough for an optimal network for the buildings to be within the catchment area: they need to be as close as possible. Obviously, this will involve all sorts of (very realistic) compromises, but that is part of the fun of the game.