Hello @Gael Regina,
Thank you for posting your query on Microsoft Q&A.
Regarding the cross-tenant synchronization between Tenant A and Tenant B, you have set the ShowInAddressList attribute to true on both sides. However, you are experiencing an issue,
- You can see users from Tenant B in Tenant A.
- You cannot see users from Tenant A in Tenant B.
To troubleshoot the issue with your cross-tenant synchronization setup between Tenant A (Exchange Online) and Tenant B (Exchange On-premise Hybrid).
To resolve this issue, you can try the following steps
- Check the ShowInAddressList attribute: Double-check that the ShowInAddressList attribute is set to true for all users in both Tenant A and Tenant B.
- Please cross check your configuration with Configure cross-tenant synchronization - Microsoft Entra ID | Microsoft Learn
- Check for any synchronization errors and review the synchronization logs to see if there are any errors log. If you find, please reach us back.
Note: Additional information – even after configuring cross-tenant synchronization, users from Tenant A will not sync with the on-premises server if you are using AD Connect in Tenant B
I hope this information is helpful. Please feel free to reach out if you have any further questions. If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".