A blacklist check is a comprehensive search of several anti-spam databases to determine if a particular IP address has been blacklisted. Anti-spam databases, or blacklists, are a common method of preventing the delivery of spam email messages. The lists consist of IP addresses either known or believed to be sources of spam. All IP addresses in a blacklist are blocked from sending email.
No notice is sent when an IP address is added to an anti-spam database. For that reason a blacklist check is often the only way a domain owner can be made aware of inclusion in the blacklist database. Some listings can be permanent, while some databases put expiration dates on blacklisted addresses.
There are dozens of anti-spam databases in existence. A blacklist check primarily consists of a search of these databases for a particular IP address. In order to conduct the blacklist check, the IP address of a remote mail server is converted into a domain name, using the ip4r format. That name is then entered into the search box for each database. Upon completion of the search, a list of the databases where the IP address in question is being blocked is then compiled by the individual who performed the search.
A blacklist check can either be performed by the individual domain owner on a blacklist search site or by a company that offers blacklist monitoring services. There are several websites that perform blacklist checks. Some of them are free, while others have a monthly fee. Many of the pay sites offer free trials. Companies that provide daily blacklist monitoring typically offer services for a monthly fee.
Inclusion on a blacklist database means that the mail server is being blocked by DNS-based Blackhole List (DNSBL) filters. Not all domains associated with a banned IP address are sources of spam. A blacklist check gives a domain owner the ability to research why a particular IP is not sending messages.
A domain can be blocked simply because its IP address is home to known spammers. In other cases, a spammer could have infected a machine without the knowledge of its owner, so that it automatically sends out unsolicited emails. A misconfigured mail server that sends out incorrect information could also lead to inclusion in a blacklist database.