This email notification problem is now solved !
Here is the explanation, if you're interested:
1) first, my web hosting company had forbidden my website to send emails, because too many emails had gone to bad addresses, making me look as a spammer. I didn't receive any warning, and the way to make it work again was well hidden...
2) when I've fixed the problem above, the notification still didn't work, because I had messed with the forum settings (when trying to fix the problem). I didn't remember it, but I had set "E-mail package size" to 1, and now that I've set it to 0, the notifications finally work !