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. You can find our list of public IP address at this page:
Please refer to your mail server or firewall documentation on how to block or allow connections based on IP address. If your mail is hosted on a shared server with other domains, you may not be able to block connections by IP addresses.