Anti-Spam Technical Reference - ZyXEL Communications ZyWall USG20-VPN User Manual

Table 175 Configuration > UTM Profile > Anti-Spam > DNSBL (continued)
LABEL
Status
#
DNSBL Domain
Apply
Reset

28.8 Anti-Spam Technical Reference

Here is more detailed anti-spam information.
DNSBL
• The USG checks only public sender and relay IP addresses, it does not check private IP
addresses.
• The USG sends a separate query (DNS lookup) for each sender or relay IP address in the e-mail's
header to each of the USG'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 USG waits until it receives at least one reply for each IP address.
• If the USG receives a DNSBL reply that one of the IP addresses is in the DNSBL list, the USG
immediately classifies the e-mail as spam and takes the anti-spam policy's configured action for
spam. The USG does not wait for any more DNSBL replies.
• If the USG receives at least one non-spam reply for each of an e-mail's routing IP addresses, the
USG immediately classifies the e-mail as legitimate and forwards it.
• Any further DNSBL replies that come after the USG classifies an e-mail as spam or legitimate
have no effect.
• The USG records DNSBL responses for IP addresses in a cache for up to 72 hours. The USG
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 28 Anti-Spam
DESCRIPTION
The activate (light bulb) icon is lit when the entry is active and dimmed when the
entry is inactive.
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 USG.
Click Reset to return the screen to its last-saved settings.
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