It was because of amavis
I activated firewall after installation of ISPConfig. For this I repead I have followed the perfect setup for Suse 10.0.
However I have found the reason of this:
cansado postfix/postfix-script starting the Postfix mail system
cansado postfix/master daemon started -- version 2.2.5, configuration /etc/postfix
cansado postfix/qmgr 74A6420047: from=<email@example.com>, size=687681, nrcpt=1 (queue active)
cansado postfix/smtp connect to mail.elmouritania.com[126.96.36.199]: Connection refused (port 25)
cansado postfix/smtp 74A6420047: to=<firstname.lastname@example.org>, orig_to=<root>, relay=none, delay=33904, status=deferred (connect to mail.elmouritania.com[188.8.131.52]: Connection refused)
This simply was because of amavis, which was not started for a strange reason in Suse 10.0 and 10.1. It complained of not FQN. So I had to refer $myhostname in /etc/amavisd.conf.
So this was nothing to do with ISPConfig - I repeat which is new for me.
So, I guess, Falko, you win some euros here - look for future comm when I will get some time to clean the 10.1 I have installed.