Transparent AntiSpam solution in ISP scale

Omid Kosari asked:

We have and ISP with adsl , broadband etc subscribers (average 10000 online users with 1M of Bandwidth . average ) . Some users infected by spammer viruses which makes our ip address spaces blacklisted in RBLs . I am looking for a transparent solution without need any change at client side.

average mail traffic is about 3Mb .

My answer:


Block outgoing port 25 at your egress, except for traffic to/from your own mailserver, and business customers who request it to be unblocked. Then require customers to relay all their mail through it.


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