Caspios emails are being delivered as unauthenticated emails to recipient host (gmail in particular).
reconfigure your email servers (spf, dkim, A & MX Records) and associate the tcp/ip port (usually 25) with the domains ip address. You will also need to configure reverse dns to authenticate the email sender to the receiver host. It's not at all difficult to do.
Could you provide some links that has more details of how to do those steps? when you mention domain's IP address...is that for your site or caspio?