WHM is a tool that hosting providers use to manage customer accounts. It was recently brought to our attention that WHM has a "Configure Remote Service IPs" setting that might have an effect on using external spam filters. For more information, see the following page in the WHM documentation:

https://docs.cpanel.net/whm/ip-functions/configure-remote-service-ips/

We do not know what effect this setting may or may not have on our service. If anyone has experience they can share with us on what this setting is for and how it has helped you in setting up your SpamHero account, we'd love to hear about it.