Email Routing - 550 Upstream error

While testing email routing - I’ve notice one problem. From my company’s email I’m not able to send the email to the routed email address. is not accepting the email. Getting error message:
Remote Server returned ‘521 550 Upstream error’”

But strange thing is - it works fine when send from different email address/server. Even more strange - this is MS Exchange online, from another MS hosted email (but M365) it works fine.

Could you please share the domain you are using?

We are seeing a few email delivery rejection from the upstream:

5.7.1 Our system has detected that this message is likely unsolicited mail. To reduce the amount of spam sent to [...],  this message has been blocked

ALL emails sent from are getting rejected. All the others end up in spam on GMail.
I have tried to send a real email I received on my work email, not just some “test” emails.

Hi @sven2 - any news? were you able to find anything ?