Email Routing fails to forward as emails are getting greylisted

I am trying to set up forwarding, however, Cloudflare fails to forward my emails as they are getting “greylisted” by my mail server which is fastmail.

So fastmail has a feature where I can enter an SMTP hostname of forwarding server (in this case Cloudflare) and it would parse the “Received headers” of this host to “find the true email source” so I am wondering if it is possible to use this feature with Cloudflare.