ZyXEL Communications USG40 User Manual page 491

Figure 324 DNSBL Spam Detection Example
IPs: a.a.a.a
b.b.b.b
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The ZyWALL/USG receives an e-mail that was sent from IP address a.a.a.a and relayed by an e-
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mail server at IP address b.b.b.b. The ZyWALL/USG sends a separate query to each of its DNSBL
domains for IP address a.a.a.a. The ZyWALL/USG sends another separate query to each of its
DNSBL domains for IP address b.b.b.b.
DNSBL A replies that IP address a.a.a.a does not match any entries in its list (not spam).
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DNSBL C replies that IP address b.b.b.b matches an entry in its list.
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The ZyWALL/USG immediately classifies the e-mail as spam and takes the action for spam that you
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defined in the anti-spam policy. In this example it was an SMTP mail and the defined action was to
drop the mail. The ZyWALL/USG does not wait for any more DNSBL replies.
Here is an example of an e-mail classified as legitimate based on DNSBL replies.
Chapter 29 Anti-Spam
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a.a.a.a?
b.b.b.b?
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DNSBL A
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DNSBL B
DNSBL C
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