This is the spamd process. It's a problem with forking and child processes (yadda yadda yadda.) An additional problem is the large amounts of memory each spamd perl process begins to use.
Lazy fix: use cron to restart spamd once a day.
Better yet: Use flags in /usr/local/etc/rc.d/sa-spamd.sh to limit the number of children spawned and the number of connections each child will handle prior to exiting. Look for the spamd_flags line and change it to something like this: