This is awhole industry.
Many times, a previous user causes an IP to be blacklisted, or a server that gets hacked or is used to advertise SPAM, etc, can be reasons for an IP to be blacklisted.
In order to get this sorted out, you must first check if its blacklisted, then there are ways to get it whitelisted directly from the RBL's site itself. Sometimes it will take an admin (aka the owner of the IP) to get involved, to get this done. Good luck.