I have been trying to resolve this with Microsoft support but I am not getting anywhere. I am setting up Exchange Online Protection for On-Prem Exchange and most of configurations have had no issue but I have started to receive two error messages:
#550 5.7.64 TenantAttribution; Relay Access Denied - This is when i email a non Office 365 tenant
Surprised MS support could not answer this one. The smart host target server name you enter in your on-premises send connector must match the MX value as shown in the DNS settings values in Office 365.
Thanks for the reply. Finally figured it out. I was using our CSP portal to access our clients Exchange Admin center (so I didn’t need to log into each tenant’s Global admin account individually). Within here I was creating the send\receive connectors and everything looked good but did not work. However when I logged directly into our clients tenant with the Global Admin account none of the send\receive connectors were there. I created them again from within the clients tenant and everything working