Problem
When sending an email, it bounced and returned a Non-Delivery Report (NDR) with the following error:
Error: 550 5.7.64 TenantAttribution; Relay Access Denied.
[DU6PEPF00009527.eurprd02.prod.outlook.com 2024-03-25T15:46:02.097Z08DC4B4F482A0112]). A non-delivery report (NDR) message was sent to zzz@yyy.com. The NDR might provide more details about why the email message wasn't delivered and how to fix the issue.
Root Cause
Microsoft began tightening inbound mail processing in late 2023, enforcing stricter rules on messages with null senders or missing return-path headers. The rollout, originally planned for November, was delayed but recently completed for most tenants. The issue commonly occurs with automated messages such as Out‑of‑Office replies, which often lack a return-path. This does not impact regular emails with proper sender information.
Solution
Using a tenant-specific certificate ensures correct attribution on Microsoft's side and prevents the relay denial.
For step-by-step instructions, see our KB article on moving to a unique certificate.