A couple of pointers for anyone wanting a cheap (as in free) anti-spam solution for Domino R5 servers.
You can take R5 up to ND6, in terms of anti-spam capability, by using DNS black hole type anti-spam software ahead of the Domino SMTP service. I recently setup an R5 server with BCware's NoSPAM SMTP proxy daemon, a free DNS black hole spam filter, and it works just fine.
I have a client who was just inundated with spam and is using Domino R5. It's a small shop that didn't have a lot of money to spend so I had to come up with something inexpensive and simple. The solution was to install BCware's smtp proxy daemon software on a separate Windows box that was collecting dust in a corner. It's probably possible to put it on the same box as the Domino server but I just wanted this to be simple. I pointed the Internet gateway router smtp forwarding to the filter box and away it went, took me all of ten minutes to set up. For security, and to block the bulk of any spam forwarding exploits ahead of the Domino server, I also installed ISS's BlackICE for Servers on this box and the Domino server itself. This cost a few pesos but does help me and my client sleep better. I have no idea how secure BCware's product is in terms of various SMTP or buffer overflow exploits but I've had no problems to date.
Here's a great IBM Redbook on the subject. The Lotus Domino 6 spam Survival Guide. It states it's for R6 but a lot of the material applies to R5 as well.