Remote Server returned '550 5.7.750 Service unavailable. Client blocked from sending from unregister

[PT_Ashraf]

We have hybrid exchange environment by using exchange 2019 CU11 & Office365

On sudden,  yesterday   onwards on-premise exchange users email is rejected with error message.

All other mail flow is working as normal

Only on-premise exchange users email is rejected from online exchange with below error. If anybody able to support ,it will be very helpful

 AM5EUR02FT092.mail.protection.outlook.com rejected your message to the following email addresses:

kfupm-IT (kfupm-IT)@kkff.edu.sa
Your message wasn't delivered because the recipient's email provider rejected it.

AM5EUR02FT092.mail.protection.outlook.com gave this error:
Service unavailable. Client blocked from sending from unregistered domains. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. (AS2157) [AM5EUR02FT092.eop-EUR02.prod.protection.outlook.com]

Diagnostic information for administrators:

Generating server: MailBox2019-1

kfupm-IT
AM5EUR02FT092.mail.protection.outlook.com
Remote Server returned '550 5.7.750 Service unavailable. Client blocked from sending from unregistered domains. For more information please go to http://go.microsoft.com/fwlink/?LinkId=526653. (AS2157) [AM5EUR02FT092.eop-EUR02.prod.protection.outlook.com]'

[#2]

Hello,

5.7.750 Client blocked from sending from unregistered domain: The error occurs when a large volume of messages are sent from domains that aren't provisioned in Office 365 (added as accepted domains and validated).

https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/non-delivery-reports-in-exchange...

can you check if something changed in the accepted domains of the tenant?

[#3]

All of the domains which are authorized sent have listed in accepted domain.
We did not sent large volume of email.

[#4]

Hi 

The issue is not fixed yet.

The incident happened on 12th October and  we opened multiple case from Microsoft  with severity level A. Still no proper support and service is down in last 6 days

[#5]

Hello, do you still have this issue?