Interesting approach - for no good reason other than it's a fairly aggresive scheme, I'd shy away from that many forks - you'll feel the impact on that box of 400 processes sending mail. (So, too, will your mail daemon!)

That said, I suspect you're hitting a specific limit of the OS and/or use that your CGI is running as. I suggest adding code to complain if the fork fails and see if that offers any clues.

if ($pid = fork) {
  # I'm the parent
} else {
  if (! defined($pid) ) {
    die "Can't fork: $!";
  }

  # Child here
}
[download]

Peace,
-McD