Fixing an issue with blocked email forwards to Fastmail from Cloudflare

I'm setting up a dev agency to do some freelance work and I wanted to route the emails to my Fastmail inbox, this will allow me to keep all my emails within one place and it's all included in the plan I have with Fastmail.

I set up the email with Cloudflare to route all emails sent to that domain but I ran into an issue that was a first for me, below is a cleaned up version of the error:

Unknown error: transient error (451): 4.7.1 <DATA>: Data command rejected: is too new, temporarily blacklisted - RLR813 - ip=<>, host=<>, helo=<>, from_domain=<>

After digging through some forums, I realized it's not a Cloudflare issue but instead an error thrown by Fastmail. In short, Fastmail will reject any domains not added to its Domains list (Settings → Domains). So setting up the DNS records in Cloudflare allowed me to do this.

Unfortunately, I have not yet found a way to route all emails through Cloudflare and then redirect them to Fastmail.