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4 

Use ‘peer‑to‑peer’ blocking services. These take advantage of humans being good at 

identifying spam and then notifying a central server which keeps an index of all spam. 

CloudMark (

www.cloudmark.com

), a peer-to-peer solution, requires users to identify 

spam by pressing a ‘Block’ button in Outlook Express which then updates a central server, 

so when others download the same message at a later time, it is automatically identified as 

spam. This technique is represented by Figure 11.30(b).

5 

Use blacklist services. 

Blacklists

 are lists of known spammers such as those reported 

to Spamhaus Project (

www.spamhaus.org/

) or SpamCop (

www.spamcop.net

). They 

Figure 11.30

Progression of attempts to combat spam

ISP

or

company

firewall

'Whitelist

check'

(c)

Closed

inbox

Wanted email

Unwanted email

ISP

Filter

+ washers

'Blacklist

check'