@Falko: shere genius. I may have found the glitch: not having included:
I just reinstalled postfix and already I can see it's a different version: postfix-2.3.3-2.1.centos.mysql_pgsql.x86_64.rpm as opposed to postfix.x86_64 2:2.3.3-2.1.el5_2.
Also postgresql.x86_64 0:8.1.21-1.el5_5.1 got installed too.
I understand; I reinstalled the correct postfix binary, I will update ISPconfig ASAP.
Wish me luck!