ZyXEL Communications 91-009-073003B manual 580

Chapter 34 Anti-Spam

Here is an example of an e-mail classified as legitimate based on DNSBL replies.

Figure 332 DNSBL Legitimate E-mail Detection Example

IPs: c.c.c.c

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d.d.d.d

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c.c.c.c?

d.d.d.d?

d.d.d.d Not spam

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DNSBL A

DNSBL B

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DNSBL C

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1The ZyWALL receives an e-mail that was sent from IP address c.c.c.c and relayed 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.

2DNSBL B replies that IP address d.d.d.d does not match any entries in its list (not spam).

3DNSBL C replies that IP address c.c.c.c does not match any entries in its list (not spam).

4Now that the ZyWALL has received at least one non-spam reply for each of the e- 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.

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	ZyWALL USG 50 User’s Guide