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Originally Posted by ianathuth
Many spammers use dictionary attacks which have more successful hits on a large ISP than a smaller ISP. They may send out millions of messages all of which have to be handled by the ISPs whether they are valid or not. Think about it, a million guesses at the part of the address before @ntlworld.com will result in more identified actual email addresses than a million on a small domain such as @chopsmcpsdomain.com
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I know - but I don't think dictionary attacks are central to the problem. Looking at the CCs in the spam I get, and analysing the fake bounces coming my way, the ntl addies are too complex and specific for that to be the case. Spammers have clearly got an enormous amount of valid or once-valid ntl addies and are hitting them repeatedly.
That said, even a dictionary attack is more profitable on an ISP with no server side spam filtering, as you know the valid guesses will reach the end-user.