In the original LBB version ship construction [both Book 2 and High Guard], there wasn't any room in a 100 dton hull for a maneuver drive after figuring in the tonnage for a 'mail distribution system'. The mail system tonnage was a complete guess-timate, as there was no reference as to how much server space terabytes of X-mail would take up. Given that the X-Boat system was originally designed in the era where home computers were flipping over cassettes and having 5 and quarter floppy drive was high-speed/low-drag, the designers opted for 'a lot' and left it at that.
So, because the X-boat itself needed to retrieved from wherever it popped in from J-space, refueled, the crew rotated, and the data-packets exchanged, the designers came up with a ship that could act as a mobile 'Pony Express station' in space. XBT's can do light repairs, refuel, bring spare crewmen, and bring spare X-boats to back to base. For an information system designed in 1977, it's worked surprisingly well.
As for making a stationary mail station, that's not a bad idea except that astrogation for J-space isn't that precise. It takes accurate charts and a VERY experience navigator to get the same jump course to arrive within hundreds of klicks of your position the last time you tried it. Since the X-boats don't have maneuver drives, the 'postal station' has to come to them.