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Fix Hybrid Exchange Email Showing Sender GUID for Microsoft 365 Users
Question details
In a hybrid Exchange environment, emails sent from specific on-premises users display a GUID or strange characters instead of the sender's name when received by Microsoft 365 users.
- Product
- Microsoft Exchange Hybrid / Microsoft 365
- Device & OS
- not provided
- Scenario
- An on-premises Exchange user sends an email to a Microsoft 365 recipient within the same hybrid organization.
- Observed behavior
- The sender's name appears as a GUID in the From field. Running Get-Recipient in Exchange Online PowerShell returns a GUID for the Name and an all-zero ExchangeGuid (00000000-0000-0000-0000-000000000000).
Ensure you have administrative access to your on-premises Active Directory, Microsoft Entra ID (Azure AD) Connect server, and Exchange Online PowerShell.
Verify and Correct Active Directory Sync Attributes
Check and update the affected user's Exchange attributes in the on-premises AD to ensure they synchronize correctly to Microsoft Entra ID.
When an on-premises user appears as a GUID in Exchange Online, it typically means their recipient object is malformed or improperly synced. Fixing the source attributes in the on-premises Active Directory is the most effective way to resolve this.
Log in to your on-premises domain controller. Open 'Active Directory Users and Computers' (ADUC) and enable 'Advanced Features' under the View menu to make the Attribute Editor visible.
Right-click the affected user, select 'Properties', and navigate to the 'Attribute Editor' tab. Verify that attributes such as UserPrincipalName, proxyAddresses, mail, targetAddress, legacyExchangeDN, and msExchMailboxGuid are correctly populated.
After correcting any missing or mismatched attributes, open PowerShell on your Microsoft Entra ID Connect server and run 'Start-ADSyncSyncCycle -PolicyType Delta' to force a sync.
Wait for the sync to complete, then use Exchange Online PowerShell to run 'Get-Recipient -Identity user@yourdomain.com' to confirm the Name and ExchangeGuid are now correctly populated.
Review Email Message Headers for AuthAs Status
Analyze the message headers of the affected emails to confirm how Exchange Online is treating the sender's authentication status.
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Frequently Asked Questions
Why does the ExchangeGuid show all zeros in Exchange Online PowerShell?
An all-zero ExchangeGuid (00000000-0000-0000-0000-000000000000) typically indicates that the on-premises mailbox was not correctly provisioned as a MailUser in the cloud. This usually happens due to missing, invalid, or conflicting attributes during Microsoft Entra ID directory synchronization.
Why do emails sent to external domains show the correct sender name instead of a GUID?
When sending to external domains (like Gmail or Yahoo), the email is routed outside your hybrid organization, and external mail systems rely purely on the standard SMTP headers. Internal hybrid routing relies on finding the matching Active Directory object; if the recipient object is broken or improperly synced in Exchange Online, it falls back to displaying the object's GUID.
What does 'X-MS-Exchange-Organization-AuthAs: Internal' mean in the message header?
This specific message header indicates that the Exchange server recognizes the sender as an authenticated internal user within your organization. This confirms that your hybrid mail flow and routing connectors are working correctly, pointing the issue directly to a malformed directory object rather than a routing failure.
