Hi All,

we run a 3rd party system that sends emails into Office365.
We authenticate with O365 SMTP servers using our SSL certificate name on an inbound connector.

Occasionally we are seeing the following error message.

550 5.7.64 TenantAttribution; Relay Access Denied

If the email is resent the message will be delivered correctly.
Does anyone have any indication as to the cause or meaning of the error message? 

• Moved by Jason Johnston [MSFT]Microsoft employee 17 hours 28 minutes ago Not a development question.

I recommend you open a ticket with Microsoft Online Support.  It looks like a problem with their service to me.