(it''s probably easier to understand it by viewing the web page rather than reading my cut & paste) "OK, now we are in the bear pit. We need to add a couple of definitions at the top of the exim.conf file as follows : Code block hostlist filter_relays = net-lsearch;/etc/filterrelays : net-lsearch;/etc/relayhosts domainlist filter_domains = lsearch;/etc/filterdomains Then, we are looking for the check_recipient: block and shortly after that there should be a line : accept hosts = : Then, following this line, you can add the following : Code block deny !hosts = +filter_relays domains = +filter_domains message = Please use the proper domain MX record"