ZyXEL Communications ZyWALL USG 300 User Manual page 616

Chapter 35 Anti-Spam
If the ZyWALL receives conflicting DNSBL replies for an e-mail routing IP address,
the ZyWALL classifies the e-mail as spam. Here is an example.
Figure 425 Conflicting DNSBL Replies Example
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The ZyWALL receives an e-mail that was sent from IP address a.b.c.d and relayed
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by an e-mail server at IP address w.x.y.z. The ZyWALL sends a separate query to
each of its DNSBL domains for IP address a.b.c.d. The ZyWALL 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
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spam).
While waiting for a DNSBL reply about IP address w.x.y.z, the ZyWALL receives a
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reply from DNSBL B saying IP address a.b.c.d is in its list.
The ZyWALL immediately classifies the e-mail as spam and takes the action for
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spam that you 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 does not wait for
any more DNSBL replies.
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IPs: a.b.c.d
w.x.y.z
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2
a.b.c.d?
w.x.y.z?
a.b.c.d Spam!
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ZyWALL USG 300 User's Guide
DNSBL A
DNSBL B
DNSBL C