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.

View the full question and any other answers on Server Fault.

Creative Commons License
This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.