Email Routing to xmhahamail.com and menghamail.com

Hello, in last 2 days we noticed mails towards the 2 mentioned domains not delivering due to the following smtp response:

421 4.3.0 Upstream error, please check https://developers.cloudflare.com/email-routing/postmaster for possible reasons why. VVNhHjZl0OJU

Could you kindly check what could be the issue?
Cloudflare Docs
Reference page with postmaster information for professionals, as well as a known limitations section.
Was this page helpful?