The stock rspamd package provided by Raspbian (jessie) is at version 0.6.10.  The rspamd repository doesn't offer binaries for the armhf architecture (e.g., Raspberry Pi 3) in order to get a more recent package.  Instead, use the yunohost repository for rspamd package (currently at 1.3.5) on armhf.

Use a Unix socket instead of a TCP socket as a Unix socket doesn't play nicely with postfix running
smtpd in a chroot. The author of rmilter recommends using a TCP socket per
https://github.com/vstakhov/rmilter/issues/39

The rmilter package changed the name of its listener service similar to
what was done for rspamd (see commit 2dbc976b).