Send & Receive Issues

Fix One User Not Receiving Email from a Partner Organization in Exchange

Ayan Masood Sep 28, 2026 869 views

Question details

Only one specific user in the organization is failing to receive emails from a partner organization sent via a TLS inbound connector.

Fix One User Not Receiving Email from a Partner Organization

Product: Exchange Online
Device & OS: not provided
Scenario: Receiving emails from a partner organization via an Exchange Online inbound connector.
Observed behavior: Other recipients successfully get the messages, but one user receives nothing, and messages destined for this user do not appear in Threat Explorer.

Before you start

Ensure you have Exchange Administrator credentials and verify that the sender has correctly addressed the emails to the exact primary SMTP address of the affected user.

Solution 1Recommended

Run a Message Trace and Check Mailbox Settings

Use a message trace to pinpoint where the email drops, and verify the user's specific mailbox rules or blocks.

Running a message trace is the most effective way to determine if the message ever reached your tenant and what happened to it afterward. Since only one user is affected, local mailbox settings are a highly probable cause.

Run Message Trace

Open the Exchange admin center, navigate to Mail flow > Message trace. Enter the partner's domain as the sender and the affected user's email address as the recipient to check for delivery status.

Check Mailbox Rules

Have the user or an admin check Outlook or Outlook on the web (OWA) for any inbox rules that might be forwarding, moving, or deleting the partner's emails.

Review Blocked Senders

Verify the user's Junk Email settings to ensure the partner's domain or specific email address hasn't been accidentally added to the Blocked Senders list.

Check Mailbox Capacity

Ensure the user's mailbox storage quota is not full. A full mailbox will silently reject incoming messages.

Run a Message Trace and Check Mailbox Settings

Threat Explorer Note: Since the messages do not appear in Threat Explorer, they may be dropping at the transport level before reaching security filtering, or they were never accepted by your tenant.

Solution 2

Inspect Inbound Connector and Transport Rules

Ensure no organization-wide transport rules or connector misconfigurations are explicitly dropping emails for this specific user.

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Frequently Asked Questions

Why would only one user stop receiving emails from a partner organization?

This usually happens due to user-specific configurations, such as accidental inbox rules, the sender being added to the user's Blocked Senders list, or the user's mailbox exceeding its storage limit.

What does it mean if an incoming email doesn't appear in Threat Explorer?

If an email isn't in Threat Explorer, it likely never reached the Microsoft Defender for Office 365 filtering layer. This points to a failure at the initial connection (such as a TLS mismatch) or the email being dropped by an edge transport rule before security scanning.

How do I check if an Exchange Online mailbox is full?

Administrators can check mailbox usage in the Microsoft 365 admin center under Users > Active users by selecting the user and viewing the Mail tab. Users can check in Outlook by clicking File > Info to view their Mailbox Settings.

Can an inbound connector block just one user?

Typically, inbound connectors apply to domains rather than individual users. However, associated transport rules tied to the connector might be configured to drop or redirect messages for specific recipients, which is why checking both is important.

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