By default qmail under Plesk is configured to allow an unlimited (!) number of processes and has no limits on the number of processes per IP or load limits. Spam runs can easily cause servers to spawn hundreds of qmail processes and make the load go through the roof. It is possible to configure various limits (processes, per_source, max_load) in /etc/xinetd.d/smtp_psa and /etc/xinetd.d/smtps_psa manually, but it would be very nice if this could be configured through Plesk's web interface. Maybe some sane defaults (instead of unlimited) would be a good idea as well.