The Spam Experiment

Tuesday, January 04, 2005

Why doesn't blocking email addresses work?

Someone on a spam newsgroup recently asked about why blocking certain email addresses doesn't work. They also asked about why we can block ip addresses without blocking legitimate email.

> Blocking IPs seems dangerous. I mean, in the states where AOL reigns, how
> will it recognize spammers from regular users??

The problem isn't AOL users.

The problem is a spammer who takes hold of someone's home computer. Someone on a DSL modem that has no firewall. The latest email worms usually contain their own SMTP engine so they can propogate themselves. Some of them also contain backdoors which allow spammers to then use the infected computer to send out spam. In fact one of these worms actually installed a hacked copy of WinProxy that contained a back door that allowed the spammer to come in and use it to relay spam.

John Doe with his unprotected cable modem has no reason to be sending thousands of emails a day. If John Doe was sending thousands of emails a day he would be sending them through a mail server. His windows 95 home PC without a real mail server should not be originating email. This is what is considered an open proxy.

There are other variations of proxy servers out there. Corporations who have misconfigured web proxy servers. These proxy servers actually allow a spammer to send email through the web proxy and hide their originating ip address. They can spoof any email address they want. Such as AOL. That is what is confusing you.

Blocking AOL or specific email addresses at AOL won't stop spam.
What will stop spam is blocking the spammers at the source. The proxy servers they use to send spam. Again a proxy server is NOT a mail server. It should not be sending spam. If it is originating email then there is a very high chance that email is spam. There is almost zero chance that email is a legitimate email.

This is one of the ways the most effective spam software works. They build open proxy lists. They don't use rbl's of misconfigured mail servers any more. The problem with using those is that you block legitimate mail along with the spam. With open proxy lists there is no reason for those systems to be sending email. They are proxy servers intended to be a proxy for web, ftp, etc. They are not intended to be a mail server.
Hope this helps!
Ben Fitts

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