Doing a DNSBL lookup on a message at SMTP connect time is cheap in hardware cycles and system time. Your DNS server may even have it cached from the last time the spammer tried.
Mail rejected by a DNSBL during delivery is not silently discarded or lost. A DNSBL realtime rejection creates a delivery status notification to the sender identifying the cause of the rejection, thereby allowing troubleshooting on the sender's end.
By rejecting the connection attempt in real-time, a DNSBL also avoids any problems associated with an email system accepting delivery, closing the connection, and then trying to return the mail to a potentially forged address after the message is determined to be spam.
DNS is inherently very efficient, using minimal amounts of bandwidth. Using BL will use much less bandwidth than having to accept every spam and virus email sent to your email system. By rejecting these spam messages during the SMTP connection, no further data is sent thereby reducing overall bandwidth requirements. DNS caching by your local DNS server also prevents redundant queries from utilizing excessive bandwidth.
The use of the nsZones DNSBLs is free of charge for low-volume non-commercial use.
For professional use, internet ISPs, or hosting providers, we can provide your unlimited or dedicated realtime access to all nsZones DNSBLs from your network.
Free mirror service is provided to sites who are willing to host a public mirror (serving data to other free users).