You want to route emails from a 3rd party service via Email Security.cloud service to utilize the scanning features of the service.
You may want to route outbound email from hosted email services or SaaS providers through Email Security.cloud to take advantage of features such as email scanning, data protection rules, encryption, and traceability. This configuration is optional but recommended. To enable outbound relaying, follow the steps below in both Email Security.cloud and your hosted provider’s configuration. These steps are necessary to ensure successful delivery and to prevent emails from being blocked.
This step applies to all providers. You must register the domain you will use as the sender. It must be a domain you own; shared domains or subdomains owned by providers (for example, acme.salesforce.com) are not supported. For detailed instructions, refer to the product documentation: Manage your organization’s domains.
Add an outbound route for your hosted provider by selecting it from the available list in ClientNet. For instructions, refer to the product documentation: Manage your outbound email routes
Follow the steps for your specific hosted email or SaaS service provider.
Microsoft O365
https://knowledge.broadcom.
Google Workspace
https://knowledge.broadcom.
ServiceNow
Salesforce
Account Engagement : Add and Verify a Domain for Email Sending