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rbldnsd uses far less memory and CPU, and is much quicker in responding to queries than BIND. Those are reasons why rbldnsd is widely used for public and private mirroring of RBL zone files. A common rule of thumb is that the overhead of doing rbldnsd and rsync becomes worthwhile for mail servers that process more than 100,000 messages per day. Some RBLs imposse a minimum daily message count for allowing rsync access for local mirroring of their zone files. Some RBLs charge a subscription fee for access. Others don't. Please check with the RBL operators as appropriate.
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