Stopping spam with the Anti-Spam-SMTP-Proxy (ASSP): "It has long been clear to me that the best place to stop spam is at an organization's SMTP server. This is true for the following reasons:
Most spam has an invalid bounce address, so notifying non-delivery simply bounces to Postmaster, creating even more wasted bandwidth. Failing to notify non-delivery is a problem for false-positives. The SMTP server is the only place where spam can be stopped before entering your system.
The only possible feedback for spammers to void an address is from the SMTP server.
Spam that passes through your SMTP server into mailboxes incurs cost to your organization: storage, transmission, backup, deletion -- in all these ways spam costs you money. The only way to minimize cost is to reject it at the initial point. "