How blacklists work !?
Some email servers and spam filters reference "blackhole lists," which are lists of IP addresses of known spammers, or "spam friendly" servers. If an IP address is on the list, they won't let your email through. Anyone can setup a blacklist for their own email server, and share the list for the public to use. Not all blacklists are created equal. Smaller blacklists, including SORBS and Spam Cannibal have less influence with the bigger ISPs. Most large ISPs only use the most reputable blacklists like SpamCop , Spamhaus and URIBL . If a server gets on a blacklist such as SpamCop, any email from the server is blocked by ISPs that use the SpamCop blacklist . SpamCop and other reputable blacklists usually have a threshold, and usually delist servers after a reasonable period of time. Find out if your IP address is on a blacklist at: http://mxtoolbox.com/blacklists.aspx More details in http://smtpdeal.com