gnu-social/docker/mail/filter/rootfs/etc/rspamd/local.d/rbl.conf

rbls {
    nixspam {
        symbol = "RBL_NIXSPAM";
        rbl = "ix.dnsbl.manitu.net";
        returncodes {
            RBL_NIXSPAM_BAD = "127.0.0.2";
        }
    }
}