Generally use this website MX TOOLBOX to find out which blacklist you are at (it can also help you to troubleshoot problems with your configuration such as good practice etc. While you're at it it will point you to blacklists you are on and will provide you with links to websites were you can remove it. Usually removing your server from blacklist happens automatically after 7 days or on simple fill out form should get it done. But there are couple of blacklists which want you to pay some money to do it. It's up to you if you want to.
As a side note. You could put a server on 2nd IP and have your main gateway different ip. This will prevent your server from getting blacklisted occasionally since only legitimate emails will come from your server and the other ip won't be listed as mail server so it won't be a problem even if it happens again. Usually when you get static ip you get more then one, it's just matter of configuring your router properly to supply that (Virtual IP and port forwarding for the rescue) :-)
To cheer you up I have had 3 cases so far of spam troubles (all 3 in diff companies) and it always worked out for the best. In such cases you really get to know where to look for help and how to troubleshoot stuff. You will come out of this stronger then ever!