mailcow-dockerized/data/conf/rspamd/local.d/dkim.conf

sign_condition =<<EOD
return function(task)
        local from = task:get_from('smtp')
        if from and from[1]['addr'] then
                lastAtSymbol = from[1]['addr']:find("[^%@]+$")
                local domain = from[1]['addr']:sub(lastAtSymbol, #from[1]['addr'])
                local keyfile = io.open("/etc/rspamd/dkim/keys/" .. domain .. ".default")
                if keyfile then
                        keyfile:close()
                        return {
                                key = "/etc/rspamd/dkim/keys/" .. domain .. ".default",
                                domain = domain,
                                selector = "default"
                        }
                end
        end
        return false
end
EOD;