Google Workspace and Microsoft 365 already support ESG on incoming and outgoing emails for years. More users and organizations are required to run a gateway nowadays.

What BOTH Google and Microsoft already provide:
1. Declare your gateway IPs as a trusted source.
2. Detect the last hop prior to the gateway (finding the real sender IP).
3. Reject anything that did not come through the gateway IPs.
4. Enforce TLS on the connection from the gateway.
5. Support routing outgoing emails through the gateway.

I hope Proton can add support for these 5 features first. Thank you.