Hi,
As strace -p <runaway process> revealed
the problem was a missing Maidir/tmp directory.
Some graceful action should be taken (log, error
message in browser etc..) instead of just trying
to open "tmp/...." thousand times. Maybe using tmpfile() would be an
even better idea.
I think setup should at least make /etc/skel/Maildir/tmp.
Courier-imapd also complained about the problem, though
it was graceful (syslog and imap error), not just eating
up CPU and mem.
However, let me note that sqwebmail is very nice and fast.
Marcell