The problem:
every time a client send out mails using mailman to more than 100 subscribed account, qmail stop to work.
Analisys.
Issue is due to inetd. I have debian etch and inetutils-inetd installed.
To avoid mailman issue, the fix is:
smtp stream tcp nowait.max
where max is a vaery large number

BUT:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=451478

inetutils-inetd does not support nowait.max

you MUST use openbsd-inetd

do not use xinted as plesk do not provide /etc/xinetd.d/* files, you have to do them yourself