Free/busy lookups fail from Exchange Online to on-premises in a hybrid deployment

  • Article
  • Applies to:
    Exchange Online

Original KB number:   4456110

Symptoms

Assume that you have a hybrid deployment that has a Microsoft 365 tenant. In this deployment, contoso.com is configured as the primary on-premises domain. For on-premises mailbox users, Microsoft Entra Connect synchronizes mailbox users in the on-premises Exchange organization, and creates mail-enabled users in Exchange Online.

The TargetAddress (ExternalEmailAddress) attribute‎ value is automatically calculated as SMTP:user@contoso.com. However, for one or more mail-enabled users, the ExternalEmailAddress attribute‎ value is set to another domain, such as SMTP:user@fourthcoffee.com.

You can verify the ExternalEmailAddress attribute‎ value on a mail-enabled user from Exchange Online Remote PowerShell by running the following command:

Get-MailUser "Identity of MailUser" | fl name, *ext*

Notice that the ExternalEmailAddress value in the output is the following:

ExternalEmailAddress : SMTP:user@fourthcoffee.com

You can also verify the ExternalEmailAddress attribute‎ value from the Microsoft 365 Exchange Admin Center. To do this, locate Recipients, and then select Contacts. Double-click the mail user in question, and then view the email addresses section. Scroll down to the bottom of the page to see the external email address.

Screenshot for ExternalEmailAddress value from Microsoft 365 Exchange Admin Center.

Note

When users are synchronized from on-premises by using the Microsoft Entra Connect synchronization tool, you cannot change the external email address directly in Exchange Online. Instead, you must change the synchronized user in the on-premises organization.

Workaround

Exchange Online calculates the ExternalEmailAddress attribute value only during synchronization. If there are no changes to the email addresses of the on-premises mailbox user, the Microsoft Entra Connect tool does not synchronize that attribute value during the scheduled delta synchronizations.

Additionally, it's not expected that the TargetAddress attribute value will ever be populated on a mailbox user.

To work around this issue, follow these steps:

  1. Make sure that the TargetAddress attribute value is no longer populated for the on-premises mailbox user.
  2. Make a change to the email addresses of the on-premises mailbox user. For example, add a new email address temporarily.
  3. Perform a delta synchronization by using the Microsoft Entra Connect tool.

You should see that the ExternalEmailAddress (TargetAddress) attribute value changes back to the expected value of SMTP:user@contoso.com. After you verify this change, you can revert the change to the on-premises email addresses if you want.

Note

You must also make sure that there are no validation errors on the Microsoft Entra object that could prevent synchronization to Exchange Online. For more information about how to check the status through the Microsoft 365 Portal or the Azure Active Directory module for Windows PowerShell, see You see validation errors for users in the Microsoft 365 portal or in the Azure Active Directory module for Windows PowerShell.

Status

This behavior is by design.

More information

When Exchange Online performs free/busy lookups, it uses the ExternalEmailAddress attribute value to determine which Organization Relationship or which Intra-Organization Connector to use. Exchange Online usually calculates the ExternalEmailAddress attribute value automatically based on the primary SMTP address of the on-premises mailbox user.

If the TargetAddress attribute is populated on the on-premises mailbox user, Microsoft Entra Connect synchronizes the TargetAddress attribute and overwrites the calculated value. When Exchange Online performs a free/busy request, this request may be sent to an unexpected location.

Even if the TargetAddress attribute value is later cleared on-premises, the ExternalEmailAddress attribute value in Exchange Online does not revert to the expected value.