IP Blacklist Check
If the checked IP address was found to be sending spam emails, it will be listed in Email Blacklists.
Most Email blackists use DNS queries for checking IP addresses, because DNS queries are fast and efficient. They are also known as DNS Blacklists or DNSBL.
If an IP address is observed sending spam it will get on a black list either by manual spam complaint, by bulk notification of large Email providers or by honeypots that are designed to attract and trap spam bots.
If a listed IP address ceases to send spam, some DNBL services will eventually remove the listing after some time, others require manual delisting. Using blacklists to reject mails from listed IPs can reduce the spam volume substantially. However, DNSBL blocking is largely ineffective against modern spam techniques that use botnets to distribute spam to a large number of infected PCs. Each infected host will then send only a small number of emails thereby flying under the radar of detection algorithms.
DNSBL blocking is also quite slow. It may take between seconds and days until an IP address gets blacklisted. In this time window, enough spam mails will get through so that the effort for the spammers is worthwhile.
There is a large number of DNSBL services to choose from, but not all are equally reliable. Therefore, the trade-off between a high detection rate and a low number of false positives is not easy. This is the reason why IP Checker considers only the most reliable DNSBL lists.
DNSBL Blackhole List (Wikipedia)
Blacklist Check (MX-Toolbox)