Fix hostname lookup for rdns rbl

src/plugins/lua/rbl.lua

@@ -78,7 +78,7 @@ local function rbl_cb (task)
 	end
 
 	local sender_dns = task:get_hostname()
-	if sender_dns ~= nil then
+	if sender_dns ~= nil and sender_dns ~= 'unknown' then
 		for k,rbl in pairs(rbls) do
 			if rbl['rdns'] then
 				task:get_resolver():resolve_a(task:get_session(), task:get_mempool(), sender_dns .. '.' .. rbl['rbl'], rbl_dns_cb, k)