ZyXEL Communications ZyWall USG20-VPN User Manual page 452

Figure 303 Conflicting DNSBL Replies Example
IPs: a.b.c.d
w.x.y.z
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The USG receives an e-mail that was sent from IP address a.b.c.d and relayed by an e-mail server
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at IP address w.x.y.z. The USG sends a separate query to each of its DNSBL domains for IP address
a.b.c.d. The USG sends another separate query to each of its DNSBL domains for IP address
w.x.y.z.
DNSBL A replies that IP address a.b.c.d does not match any entries in its list (not spam).
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While waiting for a DNSBL reply about IP address w.x.y.z, the USG receives a reply from DNSBL B
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saying IP address a.b.c.d is in its list.
The USG immediately classifies the e-mail as spam and takes the action for spam that you defined
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in the anti-spam policy. In this example it was an SMTP mail and the defined action was to drop the
mail. The USG does not wait for any more DNSBL replies.
Chapter 28 Anti-Spam
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a.b.c.d?
w.x.y.z?
a.b.c.d Spam!
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DNSBL A
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DNSBL B
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DNSBL C