First thing is you need to figure out why you were black listed could be you are sending too many email blast from your internal atomic mail server or could be a real big problem with one of your end user computer is infected with spyware or is part of hackers bot net that is sending spam emails to people around the world so to fix this and get un-blacklisted narrow down the computer that’s infected clean is or if you just send so many emails out a day to a big email list with over hundreds of emails then you need to use a third party like mail chimp or vertical response so your IP does not get blacklisted again. Here are some tools below to help you.
Check to see if you are blacklisted:
Check for Malware or part of bot-net:
More Info on this matter:
Example with one server blacklisted