Server high load, imap and smtp processes multiplying maybe qmail issue

[trec]

I am running Plesk 9.5.4 MU 29 on Linux RedHat el5
Using qmail and courier-imap

Server is running smoothly (load lower than 1) until some time when the users connect to their mail accounts at the start of labor day.
The load gets really high (500, 1000, or even 2000 on the 1 minute load average)

15:41:59 up 29 days, 23:34, 4 users, load average: 175.98, 1506.01, 2415.54

and the courier-imap processes and qmail-smtp processes start to spawn.
Sometimes I had 990 courier-imap processes and 580 qmail-stmpd processes like these:

root 13252 0.0 0.0 10468 1100 ? Ds 14:22 0:02 /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/
root 13257 0.0 0.0 10468 1092 ? Ds 14:22 0:02 /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/
root 13258 0.0 0.0 10468 1100 ? Ds 14:22 0:02 /var/qmail/bin/relaylock /var/qmail/bin/qmail-smtpd /var/qmail/bin/

What I do to fix it is to stop courier-imap and qmail, then I killall all the qmail-smtpd processes and restart those services.
The server restore without problem and works fine

in the normal usage I got around:
10 simultaneous process of qmail-smtpd
80 simultaneous process of courier-imapd

I had checked the message log, and something seems to be correct, it just report the long duration of the smtp process:
Mar 17 15:12:48 xinetd[27079]: EXIT: smtp signal=9 pid=29578 duration=753(sec)
Mar 17 15:12:48 xinetd[27079]: EXIT: smtp_p333 signal=9 pid=30334 duration=623(sec)

any clue to where to look at?

Thanks