We recommend that you configure your router, firewall, or mail server to only accept incoming mail from our range of IP addresses. This prevents spammers from sending mail directly to your server, thus bypassing the MX Guardian servers. Our current list of IP addresses are:
IPv6 Address Range
You should block inbound connections on port 25 from all IP addresses EXCEPT those listed above. If your mail server hosts multiple domains, make sure all the domains are routed through MX Guardian before adding any firewall rules. Please contact us if you have any questions.