Anti-Spam Filtering - D-Link NetDefendOS User Manual

Chapter 6: Security Mechanisms
are known as DNS Black List (DNSBL) databases and the information is accessible using a
standardized query method supported by NetDefendOS. The image below illustrates all the
components involved:
DNSBL Server Queries
When the NetDefendOS anti-spam filtering function is configured, the IP address of the email's
sending server is sent to one or more DNSBL servers to find out if any DNSBL servers think the
email is from a spammer or not. NetDefendOS examines the IP packet headers to do this.
Figure 6.10. Anti-Spam Filtering
The reply sent back by a server is either a not listed response or a listed response. In the latter case
of being listed, the DSNBL server is indicating the email might be spam and it will usually also
provide information known as a TXT record which is a textual explanation for the listing.
The TXT record is not used by IP policy based anti-spam. With SMTP ALG anti-spam, the record is
part of the X-SPAM information which is optionally inserted into the body of an email.
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