Emails being rejected with valid domain (google dot com)

I have set up email routing on a number of my domains with no issues but for one every single email sent gets rejected.

The reply email telling me that it has been rejected says the following:

Remote Server returned '521 5.3.0 Upstream error, please check https://developers.cloudflare.com/email-routing/postmaster for possible reasons why. 71uig3fmBy4u'

But the Cloudflare log says this.

This looks to me to be the message coming from Google saying that the sending domain is likely spam but the email is from Google itself.

I’m really confused and wonder if anyone has any tips of taking this forward?

Hi,
Did you get any solution for this error?
Since I am also getting same error with one of my domain.
Please help me out if you have any solution for this

In short, no, unfortunately not.

This turned out to be an issue with Google. Changing the routing to my Outlook account solved the issue and now the emails are being successfully delivered.