Hi Warry,
Do you mean that the error message “550 5.4.1 Relay Access Denied” occurred when external users sending email to Office 365 account? I understand that the domain was used with a 90 days trial of BPOS some years ago. The issue persists after performed suggestions in a blog.
To troubleshoot the issue, please refer the information in the following KB article:
http://support.microsoft.com/kb/2603474
If the issue persists, to further research this issue, please provide the full non-delivery report (NDR) via private message for further research. To protect your privacy, I have sent you a private message on this. It was responded in a private message with a subject of "Information Request".
Please go to the Your details section on the right side of the community site.
Click Private messages.
Click the subject title of the response to read the message.
You can reply by using the form in that display to provide the information requested.
For your reference, Non-delivery reports (NDRs) are system messages that report the delivery status of a message to the sender. The messages are a subclass of a general message information structure that is referred to as delivery status notifications. Delivery status notifications describe three different types of situations:
•Success (2.X.X numeric codes)
•Persistent transient failure (4.X.X numeric codes)
•Permanent failures (5.X.X numeric codes)
NDRs are generated whenever a message cannot be delivered. If the computer can detect the reason for the failed delivery, it maps the reason onto a status code, and a corresponding error message is printed. (To see a list of these codes, see RFC 1891 and RFC 1893.) For NDRs, most numeric codes are reported in the form of 5.X.X and are described as permanent failures.
For the detailed information, you can refer the article:
http://support.microsoft.com/kb/284204
Thanks,
Monica Tong