Do I need the smarthost on Microsoft 365 or Google Workspace?

Almost certainly not. The smarthost exists to fix outbound deliverability for servers that send from low-trust IPs — and Microsoft 365 and Google Workspace already send from their own reputable, well-maintained IP estates.

You probably need the smarthost if: - You run a self-hosted mail server (Postfix, Exim, a mail appliance) and your mail is being spam-foldered or rejected. - You're on a small ISP or shared host whose sending IPs have poor or shared reputation. - Your ISP blocks outbound port 25, so your server can't deliver directly at all. - Your sending IP is dynamic, shared or NAT-pooled, or you lack a proper PTR/SPF/DKIM setup.

You don't need it if your mail already leaves via Microsoft 365 or Google Workspace — routing that through a relay adds nothing and complicates your setup.

If you're unsure, check whether your outbound problems are reputation-related. The smarthost is the fix for a reputation problem, not a magic switch for every delivery issue.