Microsoft has released a TechNet article demonstrating the process of the supported integration of Microsoft 365 (formerly Office 365) with a third-party email add-on service, such as Exclaimer Cloud – Signatures for Office 365.
The article shows “the best practices for how your organization can use a third-party email add-on service” by guiding through the setup steps in the Microsoft 365 (formerly Office 365) admin center in order to configure mail flow to work with an email add-on service.

Source: TechNet
The setup process described in the article is automated in Exclaimer Cloud – Microsoft 365 (formerly Office 365). This means you won’t have to manually setup the connectors, mail flow rules or certificate domain yourself – it’s all done for you in the setup wizard.
For more information on the supported integration process of Microsoft 365 (formerly Office 365) and email add-on services, read the Microsoft TechNet article “Scenario: Integrate Microsoft 365 or Office 365 with an email add-on service”.