vCheck to see if your IP addresses are listed with 80 DNS based anti-spam databases! Will your mail server be blocked by DNSbl filters?
Just because the IP is listed with a particular blacklist does not mean that you are sending spam, just that particular blacklist suggests not to accept mail from a mail server running at that IP address. Most residential Cable/DSL IP addresses that are dynamically assigned will indicate that they are blacklisted, meaning you should be sending from your ISP's mail server, not a mail server running on your own internet connection.