ZyXEL Communications USG40 User Manual page 493

Figure 326 Conflicting DNSBL Replies Example
IPs: a.b.c.d
w.x.y.z
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The ZyWALL/USG receives an e-mail that was sent from IP address a.b.c.d and relayed by an e-
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mail server at IP address w.x.y.z. The ZyWALL/USG sends a separate query to each of its DNSBL
domains for IP address a.b.c.d. The ZyWALL/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 ZyWALL/USG receives a reply from
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DNSBL B saying IP address a.b.c.d is in its list.
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.
Chapter 29 Anti-Spam
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a.b.c.d?
w.x.y.z?
a.b.c.d Spam!
ZyWALL/USG Series User's Guide
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DNSBL A
2
DNSBL B
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DNSBL C