Security Issue in Mailserver after ISPConfig installation

[bogdinator]

Hello folks,
i was going along lucky with ISPConfig for half a year but today in the morning I've received an email from our university network administrator. It states that the server I've set up after "The Perfect Setup of ISPConfig for Suse 9.3" has an open mail relay; and whether I could arrange for it to be closed please
I am a bit confused now as I thought the installation routine was quite worked out. Is it because we can send emails at the moment without requested authentication? Any help would be appreciated.

Thanks!!!