We had that happen with Cloudflare-forwarded emails on a specific mailbox. I wrestled with CF's support until I got to someone who was able to debug the issue a

We had that happen with Cloudflare-forwarded emails on a specific mailbox. I wrestled with CF's support until I got to someone who was able to debug the issue and they found out that everything was sent successfully and accepted by Google's servers, it just vanished afterwards. What I found out afterwards: using the same mailbox with a suffix ("mailbox+cloudflare@example.com") worked just fine.