[robjan]

For that to work the destination server, in this case Gmail, would have to know to deliver mail addressed to [e-mail] into your Gmail mailbox. In this example it's a limitation on the Gmail side.

Domains and subdomains are handled by DNS which is why Cloudflare can E2E proxy them. Email mailboxes are handled by an application running on a server.

[zfa]

Sorry - just had to clarify elsewhere too so I obviously wasn't clear... I meant in situations where the backend MX has a mailbox which matches to mail recipient as in the case where you're running your own mail server and would like Cloudflare sat in fonrt of it just like they sit in front of your own webserver.

Obviously if there's any recipient address trranslation in play forwarding becomes necessary.