DNSBL

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DNSBL services (Domain Name Server Blackhole Lists) or RBL (Realtime Blackhole List) are rather effective against the spam. Every service contains a list of black IP addresses of servers, used for sending spam. The services are accessed in realtime using DNS protocol.
For evaluating a message the plug-in makes a query to a list of selected DNSBL services with sender's IP address. If the IP address is listed in several black lists (the number of required hits can be changed in settings), then the message is marked as spam.

In DNSBL page you can select for checking several services from a list. Set the minimum required hits less or equal to a number of selected services.

The black lists may be used for filtering messages on the server and on the client. To disable DNSBL filtering on the client use an option "Execute DNSBL checks for headers only".