We use an on-premise installation of Exchange Server 2013 and have started today to receive an NDR when sending an e-mail to a specific user, the content of which is as follows:
BL2FFO11FD040.mail.protection.outlook.com gave this error:
TenantAttribution; Relay Access Denied
BL2FFO11FD040.mail.protection.outlook.com Remote Server returned '550 5.7.64 TenantAttribution; Relay Access Denied'
I understand from the internal user that has sent the message that we have sent messages successfully to the external domain in the past so I don't understand why we are now receiving this error.
I don't know if I can contact the team that deals with Exchange Online directly (the MX record for the domain being sent to is [domain-co-uk].mail.eo.outlook.com) but nothing has changed on our side so I don't understand why we are now having this issue.
Any advice would be much appreciated!