ZyXEL Communications ZYWALL USG 20 Manual page 537

Unified security gateway
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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 318 Conflicting DNSBL Replies Example
4
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.
ZyWALL USG 20/20W User's Guide
IPs: a.b.c.d
w.x.y.z
1
2
a.b.c.d?
w.x.y.z?
a.b.c.d Spam!
3
Chapter 32 Anti-Spam
DNSBL A
DNSBL B
DNSBL C
537

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