ZyXEL Communications 91-009-073003B manual 581

Chapter 34 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 333 Conflicting DNSBL Replies Example

IPs: a.b.c.d

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a.b.c.d Spam!

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

2DNSBL A replies that IP address a.b.c.d does not match any entries in its list (not spam).

3While waiting for a DNSBL reply about IP address w.x.y.z, the ZyWALL receives a reply from DNSBL B saying IP address a.b.c.d is in its list.

4The ZyWALL immediately classifies the e-mail as spam and takes the action for 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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ZyWALL USG 50 User’s Guide