Today (21th of March 2013) we noticed we that a lot of our incoming email was being marked as spam. Since this included many messages that were clearly not spam, we decided to investigate more deeply. We found that a third party was subverting the Spamhaus service using a routing-based attack.
First clues: Spamhaus misbehaving?
To filter out spam we use a combination of about 30 different rules. One of them is based on the Spamhaus advisory services, which are a family of DNS-based block lists (DNSBLs) commonly used to filter email. Spamhaus keeps track of servers which are sending large amounts...
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