Anti-Spam Technical Reference - ZyXEL Communications ZyWall 110 User Manual

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Table 281 Configuration > UTM Profile > Anti-Spam > DNSBL (continued)

DNSBL Domain

40.8 Anti-Spam Technical Reference

Here is more detailed anti-spam information.

• The Zyxel Device checks only public sender and relay IP addresses, it does not check private IP

• The Zyxel Device sends a separate query (DNS lookup) for each sender or relay IP address in the e-

mail's header to each of the Zyxel Device's DNSBL domains at the same time.

• The DNSBL servers send replies as to whether or not each IP address matches an entry in their list. Each

IP address has a separate reply.

• As long as the replies are indicating the IP addresses do not match entries on the DNSBL lists, the Zyxel

Device waits until it receives at least one reply for each IP address.

• If the Zyxel Device receives a DNSBL reply that one of the IP addresses is in the DNSBL list, the Zyxel

Device immediately classifies the e-mail as spam and takes the anti-spam policy's configured action

for spam. The Zyxel Device does not wait for any more DNSBL replies.

• If the Zyxel Device receives at least one non-spam reply for each of an e-mail's routing IP addresses,

the Zyxel Device immediately classifies the e-mail as legitimate and forwards it.

• Any further DNSBL replies that come after the Zyxel Device classifies an e-mail as spam or legitimate

have no effect.

• The Zyxel Device records DNSBL responses for IP addresses in a cache for up to 72 hours. The Zyxel

Device checks an e-mail's sender and relay IP addresses against the cache first and only sends DNSBL

queries for IP addresses that are not in the cache.

Here is an example of an e-mail classified as spam based on DNSBL replies.

Chapter 40 Anti-Spam

The activate (light bulb) icon is lit when the entry is active and dimmed when the entry is

This is the entry's index number in the list.

This is the name of a domain that maintains DNSBL servers. Enter the domain that is

maintaining a DNSBL.

Click Apply to save your changes back to the Zyxel Device.

Click Reset to return the screen to its last-saved settings.

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