(421): 4.3.0 Upstream error even though everything is passing 🥲

I'm getting this error even though everything is passing and the link is useless 😭 Unknown error: transient error (421): 4.3.0 Upstream error, please check https://developers.cloudflare.com/email-routing/postmaster for possible reasons why. LfJLYFMzyUy9

Cloudflare Docs

Postmaster | Cloudflare Email Routing docs

Reference page with postmaster information for professionals, as well as a known limitations section.