In front of whatever you already run.

Microsoft Exchange and Microsoft 365

Point the MX at Spamjadoo, set Spamjadoo's relay target to your Exchange server or the 365 tenant's MX, and add a connector or receive rule so Exchange trusts mail from Spamjadoo's addresses. For 365, enable Enhanced Filtering for Connectors so it reads the original sender IP from Spamjadoo's headers.

Google Workspace

Point the MX at Spamjadoo, then in the Workspace admin console set Spamjadoo's addresses as an inbound gateway. Workspace will then apply its own SPF and DMARC checks to the original sender rather than to Spamjadoo.

cPanel and WHM

For hosting providers, Spamjadoo becomes the MX for every hosted domain and relays to the cPanel server's Exim. Recipient validation is done live against the cPanel account list, so unknown users are rejected at RCPT TO rather than accepted and bounced.

Postfix, Sendmail and Zimbra

Relay to the server as usual. Restrict the server to accept port 25 only from Spamjadoo's addresses so nobody can bypass the gateway by connecting to the backend directly. Zimbra deployments use the same pattern with Zimbra's own Postfix.

Recipient verification

Every integration benefits from live recipient lookup: LDAP, Active Directory, a cPanel account list, or an SMTP callout to the backend. It is what makes directory harvest protection and backscatter prevention possible.