As the Microsoft 365 Administrator, encountering the 554 5.7.1 : Relay access denied error when sending to an external domain indicates a breakdown in the trust relationship between your MS365 tenant and the recipient’s mail server.
The “Relay access denied” error is a permanent SMTP rejection. It means the recipient’s mail server received your email but refused to “relay” it to the final mailbox because it couldn’t verify that your server is authorized to send mail to that address.
Since your mail is being rejected by Hostinger’s servers, and assuming your MS365 setup is standard, you should provide the NDR (Non-Delivery Report) to the recipient via an alternative method (e.g., phone or personal email) and ask them to verify with Hostinger why their server is denying relay for legitimate incoming mail.
If this is only happening to this one recipient, it is 99% a recipient-side configuration issue. If it happens to all external recipients, the issue is your MS365 Outbound Connector or SPF record.
Solution:
- Delete the unncessary DNS records to ensure all faulty settings are deleted.
- Re-establish the connection with Microsoft 365 again by adding the DNS settings provided inside you Admin Control Center.
- Verify SPF, DKIM, and DMARC.
The issue should be fixed as soon as the DNS changes propagate. This is not a very hard fix but identifying the meaning behind the code is time consuming.
Get in touch with us if you need help propagating these changes and your internal teams struggle to fix these issues. We will take over your IT support and give your team the bandwidth to consider what really matters for your business.