Shared mailbox not receiving external emails

Question

One of our shared mailboxes in Exchange Online. Internal messages reach it just fine, but any external emails never show up in the inbox. Message trace indicates they’re accepted by Exchange Online Protection, but there’s no sign of delivery or rejection afterward no NDR, no quarantine. Other shared mailboxes in the same tenant are working as expected, so this seems isolated.

Answer 1

Hi @Buchanan Christopher

Thank you so much for contacting Microsoft Q&A Forum.

I understand you're facing a critical issue where one specific shared mailbox isn't receiving any external emails, even though internal mail flows correctly and message trace shows Exchange Online Protection (EOP) accepted the messages without any sign of rejection or quarantine. 

To help troubleshoot this, I have a couple of questions: 

1. Were any rules (like transport rules or inbox rules) or settings changed on this specific shared mailbox recently? 
2. Can you confirm if this is happening for all external senders trying to email this mailbox, or only specific domains/senders? 

While you check on that, I've looked into this issue and found a solution that has helped many other users resolve this same behavior. It often relates to a hidden sender authentication setting on the mailbox that needs to be reset. 

You can try toggling this setting using Exchange Online PowerShell. 

Please try running these two commands in order (replacing <email address> with your shared mailbox's email): 

Step 1. Enable the setting: 

Set-Mailbox -Identity <email address> -RequireSenderAuthenticationEnabled $TRUE  

Step 2. Immediately disable the setting again: 

Set-Mailbox -Identity <email address> -RequireSenderAuthenticationEnabled $FALSE  

Reference: https://learn.microsoft.com/en-us/answers/questions/4929439/shared-mailbox-not-receiving-external-email

Please let me know if this resolves the issue for you. 

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