Using classic Outlook for Windows in business environments
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Hi James Spitz
Since only one mailbox is affected while other mailboxes in the same domain are receiving messages normally, this may be related to the mailbox itself, mail flow settings, or server-side configuration rather than the Outlook client.
To better understand your issue, please check the following:
-Send a test email from an external account to the affected mailbox and note whether the sender receives a Non-Delivery Report (NDR) or bounce-back message.
-Sign in to Outlook on the Web (OWA) and check whether new messages appear there.
If messages are missing in both Outlook and OWA, the issue is likely on the server side rather than with Outlook.
Since your thread's tag is business account, I also recommend contacting your organization's IT administrator. The administrator can perform checks that are not available to end users, including:
- Running a Message Trace to determine whether incoming messages are reaching the mailbox. (Message trace in the new EAC in Exchange Online | Microsoft Learn)
- Checking for mail flow (transport) rules that may be blocking or redirecting messages.
- Reviewing anti-spam, anti-phishing, and quarantine policies.
- Verifying the mailbox status, licensing, and quota.
- Confirming that no recent administrative changes have affected mail delivery.
If you are the Microsoft 365 administrator, please share the results of a Message Trace for a message.
I hope this information helps and look forward to your response.