How to fix Delivery has failed to these recipients or groups

Hello @Hoa Doan,   

Thank you for reaching out to the Microsoft Q&A forum! 

I truly appreciate your efforts to troubleshoot the issue on your own; I can see you've already tried most of the essential steps.   

Based on the information you provided; especially the fact that you are using WHMCS and have previously been able to send emails successfully to these recipients; it appears that the issue may be related to the WHMCS system itself, rather than Outlook or Microsoft 365. 

As WHMCS is a third-party platform, I have limited visibility and control over its internal configurations. However, I’ve tried my best to research the issue on your behalf, and here are some helpful resources that may assist you: 

You can access their official support page here: Technical Support | WHMCS. Their support team should be able to assist you further with SMTP configuration or delivery diagnostics. 

 Additionally, I recommend posting your issue on the WHMCS.Community where you'll find active discussions, shared experiences, and support from both WHMCS moderator and other users who may have encountered similar issues.  

Although your concern is typically outside the scope of my usual support responsibilities, I understand how important it is to resolve it quickly. That’s why I’ve taken the time to gather these resources for you. 

Once again, thank you again for raising this issue; it’s helped broaden my understanding. I hope you understand that I want to help as much as I can within the scope of this forum.        

Look forward to hearing from you!        

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I followed the instructions but the error still occurs

To resolve the issue of emails bouncing back with a message indicating delivery failure, you can follow these steps:

1. Verify Recipient's Email Address: Ensure that the email addresses you are sending to are correct and have not changed. A simple typo can lead to delivery failures.
2. Check Non-Delivery Report (NDR): If you received a non-delivery report (NDR), check the details in the message for any specific error codes or messages that can guide you on the issue.
3. Recipient's Mailbox Status: If the recipient's mailbox is full, you will receive a message indicating that the mailbox cannot accept messages. In this case, you can try resending your email later or contact the recipient directly to inform them.
4. Permissions: Ensure that you have the necessary permissions to send emails to the recipient. If the recipient is part of a group, you may need to be on the allowed senders list for that group.
5. Contact Email Admin: If the problem persists, it may be beneficial to contact your email administrator for further assistance. They can check for any issues on the server side that might be causing the delivery failures.
6. Resend the Email: If you have verified the email address and permissions, try resending the email. You can also attempt to send the email from a different email client or web interface to rule out any client-specific issues.

If you continue to experience issues, consider looking into specific error codes provided in the NDR for more targeted troubleshooting steps.

References:

• Fix NDR error "550 5.1.10" in Exchange Online
• "AggregateGroupMailbox is full" when sending emails to Microsoft 365 Groups