spamassassin: make it work!
I emerged spamassassin, which installed the spamd server, and the spamc client, with which to talk to the server.
However, the logs were complaing TELL commands have not been enabled. It seems that SPAMD_OPTS in /etc/conf.d/spamd is missing the -l (–allow-tell) option.
Adding the above -l to SPAMD_OPTS allowed spamc to talk to spamd and make it start learning.