UCSPI-TCP is a set of command-line tools for building TCP-based client/server applications. They are compliant to UCSPI, the UNIX Client-Server Program Interface. UCSPI tools are available for several different types of networks. For more information, please see http://cr.yp.to/ucspi-tcp.html
Installing ucspi-tcp is pretty straighforward:
# cd /usr/ports/sysutils/ucspi-tcp
# make install clean
Please make sure the SSL Protocol support box is checked.
When you run that command, you have 4 options. I would highly suggest installing the man pages. If you would like to use rblsmtp with uscpi, that is completely up to you. By experience alone, I can tell you enabling rbls will dramatically decrease the amount of spams you get. If you have or plan to have a large email server, this will definitely help in the long run.