RADIUS before SMTP solves mostly the same problem as POP before SMTP except it doesn't depend on the (sometimes) unpredictable behavior of various mail clients. I wrote this code mainly to solve the problem of one of my customers who shared a dial up pool with several other ISPs and only wanted his own users to be able to talk to his SMTP server.
RADIUS before SMTP does this nicely by using the RADIUS server to place the IP address of a user into a Berkeley DB file when the user logs in, and taking the IP address back out when the user logs out giving the SMTP server a way to limit access to only our own users.
I also plan to modify Dialin Spammer Block to read EXIM log files and work with RADIUS before SMTP (by simply removing the offending users IP from the database instead of adding it)
Dialin Spammer Block is where I originally got the inspiration for RADIUS before SMTP.
08/16/2007: First public release of the RADIUS before SMTP code an article about the software and spam control in general will be posted soon.