You've enabled DLP Cloud Service for your email solution, but when sending messages they are returning the following error in your O365 Admin Center:
451 4.4.62 Mail sent to the wrong Office 365 region. ATTR35. For more information please go to https://go.microsoft.com/fwlink/?linkid=865268 [<O365SmartHost>.eop-nam10.prod.protection.outlook.com]
This error occurs when the DLP Cloud Detector is configured to accept mail from one O365 "forest" or Tenant, but messages originate from a different one than configured.
It's most commonly a result of having 2 DLP and O365 tenants, e.g., one for PROD and another for UAT or TEST - and the Outbound and Inbound Connectors are pointing to the wrong DLP Detector.
Release : 15.8
Component : Cloud Service for Email
Verify that your Outbound and Inbound Connectors are set to send and receive mail from the correct "SmartHost" - the same one as provided in your Welcome Email from DLP Cloud Service.
The correct values are also in the Enforce Server, as the "DETECTOR FQDN" - e.g., <DetectorID-GUID>.ds.dlp.protect.broadcom.com
Instructions for configuring O365 and Gmail connectors and transport are given in the Implementation Guide: Symantec_DLP_Cloud_Service_for_Email_Implementation_Guide.pdf (broadcom.com)