Email error feedback from external

Question

Hi,

Please advise on how to resolve as per attached as my external user had feedback they will received a undelivered error message whenever they try to sent me an email.

Answer 1

Dear @Clarence Neo

Good day. Thank you for posting your question in the Microsoft Q&A forum!

I sincerely apologize for the inconvenience you’ve experienced. I truly appreciate your patience during this time and thank you for bearing with us.

Based on your description, I understand that your external user is having trouble sending emails to you and is getting an undelivered error message.

Thank you for sending over the screenshot of the error message. After carefully analyzing the details, I can confirm that the issue is not with your Microsoft 365 account but rather with the email client configuration of the external sender. 

Here are a simple breakdown of the problem and the solution. 

The key part of the error message is: is not permitted to relay through this server without authentication. 

• In simple terms: The sender's own email server (in this case, a Mimecast server) is blocking their outgoing email before it is even sent to you. 
• The reason: The sender's email client (e.g., Outlook, Apple Mail) is trying to send the email without first logging in with a username and password. Modern email servers require this login (called "SMTP Authentication") as a security measure to prevent spam. Because the sender's client isn't providing these credentials, their server refuses to "relay" or send the message. 

The fix must be applied by the external sender, as the problem is on their end. You will need to forward the following instructions to them. 

Please forward this information to your external contact: 

"Hello, 

The email you sent could not be delivered because your email client is not correctly configured to authenticate with your outgoing mail server. 

To fix this, you must enable "SMTP Authentication" in your email application's account settings. This setting typically involves instructing your email client to use the same username and password for the outgoing server (SMTP) as it does for the incoming server. 

Please contact your IT department or email service provider for assistance and provide them with the original error message. They will be able to guide you on how to enable this required setting." 

While the issue is external to Microsoft 365, this official Microsoft document explains the concept of SMTP authentication and why it is essential for modern email security. You can share this for additional context. 

• For reference: Enable or disable authenticated client SMTP submission (SMTP AUTH) in Exchange Online

In summary, your email account is working correctly. Once the sender corrects their email client settings, their messages will be delivered to you without issue. 

I hope this explanation is clear and helps you resolve the problem with your contact. 

Note: Please note that our initial response does not always resolve the issue immediately. However, with your help and more detailed information, we can work together to find a solution.   

Your understanding and co-operation are highly appreciated. Thank you for your precious time. Have a nice day!

Looking forward to your response!

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