where this email coming from? Can you write more details about the situation?
A 521 error happens when Cloudflare is unable to make a TCP connection to your origin server. Cloudflare tries to connect to your origin server on port 80 or 443, but receiving a connection refused error. This is often caused by security or firewall software and happens if the origin server has directly refused Cloudflare’s proxy request.
here some ideas to check more: