Because that would make dynamic addresses unusable? I don't have google's IP addresses memorized; do you? That said, there are lots of things providers can do to mitigate botnets. One of the main ones is outbound port 25 blocking. Most dynamic users should be using their providers' mail servers for outbound mail and not sending directly. Any legitimate mail servers should be moved to static addresses or can be allowed out by special arrangement. Many providers currently do this. More need to. That would stop a huge amount of spam, phishing and malware propagation by email.

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