ZyXEL Communications ZYWALL USG 2000 Support Notes page 170

ZyWALL USG 2000 Support Notes
3.3.1 How Anti-Spam works on ZyWALL USG
There are two ways to implement the anti-spam function. One is to make black list and white
list; the other is to refer the black list from one credited website, this function is called as
DNS-based black list (DNSBL)
3.3.2 Using DNSBL (DNS-based blacklist)
There is another way to avoid receiving spam emails. Currently, there are some websites
which are responsible for monitoring and analyzing email server activity in the network.
Accordingly, they maintain a list of spam email servers and publish this list. ZyWALL will
check the list provided by the DNSBL website to see whether it is listed in blacklist when this
function is activated.
3.3.2.1 Application scenario to apply DNSBL
There are two scenarios for demonstrating this function. One is supposed that the company's
email server is located in the DMZ and the other presents email server is located in the
ISP/Internet.
3.3.2.1.1 Scenario I: Email server is located in the ISP/ Internet
In this case, the Email server is located at ISP or Internet, which means end user will receive
the email through protocol POP3 only. It also means that the location of email server is outside
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