ZyXEL Communications ZyWALL USG 300 User Manual page 615

DNSBL C replies that IP address b.b.b.b matches an entry in its list.
3
The ZyWALL immediately classifies the e-mail as spam and takes the action for
4
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.
Here is an example of an e-mail classified as legitimate based on DNSBL replies.
Figure 424 DNSBL Legitimate E-mail Detection Example
The ZyWALL receives an e-mail that was sent from IP address c.c.c.c and relayed
1
by an e-mail server at IP address d.d.d.d. The ZyWALL sends a separate query to
each of its DNSBL domains for IP address c.c.c.c. The ZyWALL sends another
separate query to each of its DNSBL domains for IP address d.d.d.d.
DNSBL B replies that IP address d.d.d.d does not match any entries in its list (not
2
spam).
DNSBL C replies that IP address c.c.c.c does not match any entries in its list (not
3
spam).
Now that the ZyWALL has received at least one non-spam reply for each of the e-
4
mail's routing IP addresses, the ZyWALL immediately classifies the e-mail as
legitimate and forwards it. The ZyWALL does not wait for any more DNSBL replies.
ZyWALL USG 300 User's Guide
IPs: c.c.c.c
d.d.d.d
1
4
c.c.c.c?
d.d.d.d?
d.d.d.d Not spam
2
3
Chapter 35 Anti-Spam
DNSBL A
DNSBL B
DNSBL C
615