office 365 Cross tenant sync - Global address list

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The Global Address List (GAL) in Exchange Online only includes mail-enabled objects (like mail users, mail contacts, and mailboxes). Simply syncing users to Entra ID (Azure AD) is not enough, they must also be mail-enabled in Exchange Online. Guest users (B2B) are mail-enabled automatically and appear in the GAL. However, cross-tenant synced users are not mail-enabled by default and require extra steps to appear in the GAL. Here are some suggestions to help you troubleshoot and resolve the issue:

1. Confirm User Sync

First, ensure that users from Tenant B are successfully synced and visible in Microsoft Entra ID (Tenant A). Go to the Microsoft Entra admin center, select Users, and verify that the expected users are present.

2. Verify Attribute Mapping

• Navigate to Identity in Microsoft Entra ID > External Identities > Cross-tenant synchronization > Select the configuration for the target tenant and click on Mappings
• Ensure the following mappings are correct: userType is mapped to Member (not Guest), and showInAddressList is mapped to True for all synced users
• If adjustments are needed, edit the mappings and save changes. This may trigger a resynchronization.

3. Check User Status in Exchange Online if the synced users are mail-enabled

• Navigate to Recipients in Exchange admin center (EAC) > Contacts.
• Look for the synced users. If they are not listed as “Mail user”, they are not mail-enabled and will not appear in the GAL.

4. Mail-Enable the Synced Users (PowerShell Required)

Connect to Exchange Online PowerShell:

Connect-ExchangeOnline

Find the Synced Users:

Get-User -Filter {UserType -eq 'Member' -and RecipientTypeDetails -eq 'User'}

Mail-Enable each user, run:

Enable-MailUser -Identity ******@domain.com -ExternalEmailAddress ******@externaldomain.com

Replace ******@domain.com with the synced user’s UPN, and ******@externaldomain.com with their actual email address in Tenant B.

Ensure User is Visible in GAL:

Set-MailUser -Identity ******@domain.com -HiddenFromAddressListsEnabled $false

For more information: Enable-MailUser (ExchangePowerShell)

It may take several hours for the GAL to update and for the users to appear in Outlook/OWA.

For more details on syncing users and making them visible in the Global Address List in multi-tenant organizations, please refer to: Synchronize users in multitenant organizations

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