Well, I misspoke [1]. The Obligatory Email Notification still wasn't functioning properly (the actual notifications are sent fine—it's just signing up that was borked). And I suspect it's been broken since it was moved to this server (or even broken on the server previous to this one). The code was written when this site was running on tower [2].
It took a bit to find the actual source code (since it was written a long time ago) and I decided to update the code while I was at it (it was compiled against an older version of a library I wrote). Then it was a rather painful process of debugging the program, which mostly had to deal with typos and permission problems.
Sigh.
But it's now working properly.