Why does anyone use "p.s." in e-mails? I'm not judging, mind, because I still do it myself. But consider: p.s. stands for the Latin post scriptum, meaning "written after." After what? In the pen and ink days, it was after you'd signed your name but before the letter was sealed and posted. You couldn't edit a letter undetected -- unless you wanted to recopy the entire thing. And since p.s. is more often used as an "oh, by the way..." or an "I forgot to tell you..." that for anything of momentous import, you just add it at the end.
With anything handwritten going the way of tanzanite, we can all edit our missives without anyone knowing the difference. Until you hit send, everything's still up for grabs. Maybe the p.s. for the digital age would be forwarding your sent mail to the same recipient, addending it with whatever you had failed to communicate the first time.