After debugging the bloody thing for two hours at a friend's, I
discovered that the source of the problem was the DUMB Debian
maintainer, who set `nnmail-spool-file' to "/var/spool/mail" at a
00debian.el file. That setting didn't do what he expected, only he
never bothered to CHECK.
If he at least used a simple SETQ, it would have been trivial to find
it. This way the only clue that anything was wrong was provided by
`symbol-plist', after I remembered to check it.
Sheesh. What a bunch of Penguinoid losers. :-(