Same, tested from a google and bounce back saying
521 5.3.0 Upstream error, please check Postmaster | Cloudflare Email Routing docs for possible reasons why.
Cloudflare email routing log says:
upstream (outlook-com.olc.protection.outlook.com.) error: failed to initalize: Unknown error: permanent error (550): 5.7.1 Unfortunately, messages from [104.30.8.47] weren’t sent. Please contact your Internet service provider since part of their network is on our block list (S3150).