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Check to see if your IP addresses are listed with nearly 70 DNS based anti-spam databases! Will your mail server be blocked by DNSbl filters?
Probably the most common way of detecting spam is rejecting mail that comes from mail servers known (or believed) to send spam. This is done by taking the IP address of the remote mail server, converting it to a domain name using the ip4r format (a.b.c.d becomes d.c.b.a.lookupzone.com), and doing a lookup to check if that IP address is listed in one of the databases.
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 directly from 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.
If you have questions or would like to discuss DNS based blacklists please visit the DNS based Blacklists Forum.
FAQ: What is a DNSBL?