First of all, if your mailserver stopped working after you subscribed to the ISPConfig nwsletter it is a coincident. The newsletter subscription simply sends an email back where you have to click on the confirm link. You have posted this confirmation email from the newsletter subscription above.
To your spamhaus.org listing: Is your server hosted on a DSL line or in a datacenter? Most DSL IP addresses are listed at spamhaus.org and you cannot remove them from there because thy belong to the range of IP's that are listed for dynamic IP assignement (Dialup or DSL).