Have you tried clicking the links to SORBS.NET to find out why you're blocked? They're oudated now, so they won't work, but you can still look up the IP address in the link on their DNS block list lookup page.
SORBS.NET is a block list mantainer. It looks like your mail server's IP address is on their DNS block list.
According to SORBS, your IP is dynamic, and they don't like dynamic-DNS-based MX records and host addresses. If you have a static IP address, then it's because the address is in a range that's listed as dynamic. You would need to follow the instructions on the SORBS site to address the issue they have with your address and then contact them to be delisted.