To setup SpamTitan Cloud with Google G-Suite, G-Suite requires you to set your MX records to G-Suite's MX records initially. Once this is done, and Gmail is enabled, you may then change your MX records. Prior to this however, there are a few settings to be done.
First, on SpamTitan Cloud, you will want to set your Destination server your domain. Under Settings->Relay Settings, Add or Edit your domain, and set the destination server to be the MX records provided by Google, separated by commas, such as:
And you will want to set Recipient Verification to "Dynamic Recipient Verification" with the Verification server set to "aspmx.l.google.com" (or the lowest value MX record provided to you by google, if it is different).
After this, you will want to go make a change in G-Suite. Sign into G-Suite, and go into the settings for Gmail, then into Advanced Settings. Look for "Inbound Gateways" and edit it to look like so:
Note: The IP addresses may be different, it is recommended to do an A record lookup of the MX Records provided in your SpamTitan welcome email.
The "Require TLS for connections from the email gateways listed above" is optional. SpamTitan will always use TLS anyways.
After that is done, you can change your MX records to those provided in your SpamTitan welcome email (such as cloud7.spamtitan.com and cloud8.spamtitan.com).