TL;DR: Cloudflare needs to disable IPv6 that they use to communicate with the Gmail servers to stop Gmail from bouncing all routed email.
Don’t let Cloudflare support tell you that email routing to Gmail addresses is your fault. Support tried to brush me off and tell me that the problem was a result of me not turning off the sender verification in your destination mail server for any forwarded recipient domains. I detailed the whole thing in my last post here. After I never heard back from support again and 30 days had passed I had enough and took to Twitter to tweet at the Cloudflare leaders. I spend very little time on Twitter since the new ownership took over, but since Cloudflare never seems to reply on Mastodon there seemed to be no choice. Well that finally got some movement on my ticket and I heard from a Cloudflare support person.
Previously I had no luck routing email to another email service, all mail still bounced, but I had been wanting to try ProtonMail for awhile so I decided this might be a good opportunity to try ProtonMail. After setting up my account with them I setup the ProtonMail address and a confirmed destination address and moved the email routing on Cloudflare from using ImprovMX and went and sent a test email to my lgr.ca email address. To my surprise it worked! Cloudflare email routing was able to route my email to another email provider with no problem so I thought I would try to send that email to my Gmail account. Unfortunately routing that email to Gmail still bounced no matter what account I sent my test emails from.
This was now in the hands of Cloudflare support, and they finally came through. After working with them, and even doing a redirect to one of their Gmail addresses, they finally admitted that the problem was a result of something they were doing and had a fix. Why they could not set this up and find the problem without me having to route an email address I did not really use to them still is a bit of a mystery to me, but the end result is this is fixable.
It appears the whole problem with Gmail bouncing ALL email from Cloudflare routing is a result of Cloudflare using IPv6 to communicate with Gmail. Once they disabled IPv6 all email was able to be forwarded to Gmail. I have since been able to forward all my email to Gmail again and once again all seems to be working.
I don’t know if Cloudflare has made this fix for everyone, but I can tell you that since they made this change all my email has been able to be routed to Gmail and only the spam that should be refused by Gmail is being refused.
Congrats to Cloudflare support for finally getting to the bottom of this. I hope that people submitting community and support tickets for Gmail bouncing all routed email actually get fixes from Cloudflare now instead of being brushed off.