I tried to access Senderbase network and performed a check on hkcable.com.hk. Surprisingly, Senderbase returned that over 2300 IP addresses of hkcable.com.hk were put in realtime blacklists. Some IP addresses have mail volume of 4.7 - 5.1. These IP addresses were connected to compromised machines which are either zombies or open proxies.
The impact of zombies or open proxies will be reduced significantly if an ISP can barred TCP port 25 in the outbound direction. This is a worrying trend and I really hope the ISP in question can take some positive actions to reduce email spams arising from compromised hosts.
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