Invalid Undeliverable Message for Outside Senders

Question

We have an old email address & a brand new one that we just created that will send and receive to and from outside email addresses, but that person will receive a Microsoft generated undeliverable notices stating that the Microsoft email address will not allow for mail from outside of the organization. I have gone into the exchange admin settings and confirmed that it's all set to receive from anyone and has no blocked senders of any kind. Mind you, one of these accounts was created brand new yesterday. The emails will still go through both ways, but the customer receives the undeliverable message. Does anyone know how to fix this. We had Microsoft dig into it back in May and it seemed like they fixed it. It started happening again on that particular email address - none of our other email addresses are having the issue. So, we went ahead and just made an entirely new email address with no link to the other one and it started doing the same thing. I was on the phone with Microsoft this morning and they still haven't been able to figure it out.

Answer 1

Hi @Christina Hirst, 

Thank you for reaching out to Microsoft Q&A Forum. 

I completely understand how frustrating this must be, especially after previous efforts to resolve it. The fact that the issue persists even with a brand-new email address, suggests an underlying policy or configuration affecting external email handling. 

To accurately diagnose the issue, I need further information: 

• Whether both old and new email addresses belong to the same tenant. 
• Most importantly, kindly ask sender to provide a screenshot of the bounce message including NDR (Non-Delivery Report) as it will reveal the exact error message and help pinpoint the root cause. 

While you gather more details, based on my testing and research, here are some steps you can check and troubleshoot in the meantime:

• Review Mail Flow Rules, Accepted Domains, and Connectors to ensure no settings are blocking or misrouting external emails:  Go to Mail Flow -> Rules: Disable or adjust rules to test.  Go to Mail Flow -> Accepted Domains: Ensure the domain is listed as Authoritative  Go to Mail Flow -> Connectors: Ensure there are no custom connectors misrouting or rejecting external mail. 

• Even if global settings allow external mail, individual mailbox settings might override them:  

Go to Recipients -> Mailboxes -> select your mailbox, then go to Mailbox features -> Message delivery restrictions -> Uncheck Require that all senders are authenticated. 

• In case the address is part of a group, go to Recipients -> Groups, then double-click the Office 365 Group you need to modify and check the box as below: 

Kindly double-check that the recipient’s email address is entered correctly. If the issue persists, it might be worth verifying whether your email domain has any restrictions or has been flagged on external systems. 

I truly appreciate the time and effort you’ve invested in troubleshooting and hope these suggestions help alleviate the challenges you’re experiencing. 

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Answer 2

Apparently, we had it setup to forward in training and it was triggering this, but no one knows why. It's been corrected, but apparently having a forward on the email is going to cause kickbacks.