Guys i think i have discovered a bug.
Ispconfig allows you to "rename" a domain. i.e. from test.com to test2.com and this is what i did.
Although most of the features are updated fine i.e. apache virtualhosts file, the local-host-domains and virtuser files of postfix keep the previous domain setting, thus the emails are not delivered correctly.
I had to remove the website and create it from scratch in order to have all the files correctly. After that for some reason i started getting many emails and my postfix started sending emails in a loop. I removed all the accounts and deleted everything inside /var/spool/postfix... i.e. active folder.
At this moment i think everything is working as it should but guys it was a nasty problem and i am happy that is solved (at least i think).
I think the developers should look into the "renaming" case. Safest option would be after you insert a domain not to be able to change it i.e. with a non editable field.