521 5.2.1 Error: True source IP is in global black list

New Contributor

Hello Everyone,


I am new to the Exchange Online and recently we have migrated our service from G suite to Office 365 and since we migrated we are getting an error "521 5.2.1 Error: True source IP is in global black list"


and Yes, receipt domain is protected and applied IP restriction for incoming domain and asking for the list of IPs of Office 365 so that they can allow it. 


I raised a ticket with Microsoft and its been almost 15 days but couldn't get the list of IPs for our tenant.


If anyone has similar issues and somehow could get it from any where could you please help me to resolve this?


Help would be appreciated. 




2 Replies
Is the error for outbound or inbound mail?
For outbound - When we send an email to the recipient we get the above error as bounce back.