First, we can design a custom email solution with servers in your NOC. These servers can provide only spam filtering or a complete ISP-quality SMTP and POP solution for your customers.
Second, ISP's can use the SpamCop Blacklist as a standard DNS-based blacklist. While this solution won't block as much spam as a full filtering server will, it is easily integrated into an existing email system and requires no additional hardware.
Feel free to contact us and let us help you protect your customers from spam.