While working on a client’s Exchange server, I stumbled across an interesting knowledgebase article: SMTP tar pit feature for Microsoft Windows Server 2003. Tarpits insert a delay into certain SMTP communications that are associated with spam or with other unwanted traffic. They work by slowing all responses that contain SMTP protocol 5.x.x error codes. For the Microsoft SMTP server the delay value is configurable. There are some caveats as to whether or not this will be effective in your environment and whether or not it will slow legitimate traffic. It is definitely worth checking out.
On a related note, it would be interesting to put a LaBrea Tarpit on the top and bottom IPs of an Internet facing subnet. For example, if I owned subnet 184.108.40.206/24, I would bind addresses 220.127.116.11 and 18.104.22.168 to a LaBrea host. Zombie computers often do linear address space scans looking for victim machines. Having a tarpit at either end of the subnet could could significantly slow zombie activity. It would also tend to clean up much of that junk that you see in your HTTP or IDS logs.