User Access Issue

Hi, @Mahmoud Teleb

From your description, this issue is usually caused by things like auto-discovery service configuration. Autodiscover helps Outlook clients and other services find mailboxes and configure settings.

Here are some tips and steps you can try:

1. Ensure that the Exchange on-premises environment and O365 are properly configured for Autodiscover DNS records. For on-premises, the Autodiscover records should point to the on-premises Exchange server. For O365, the Autodiscover record should point to the O365 service. For more information about configuring the on-premises Exchange Autodiscover Service, see Configuring the Exchange Autodiscover Service How to Configure Autodiscover Service in Exchange Server? (stellarinfo.com)
2. Check the Service Connection Point (SCP) in your on-premises Active Directory. SCP should point the Autodiscover service to your on-premises Exchange server. More information can be found here Autodiscover service in Exchange Server | Microsoft Learn
3. To ensure proper mail flow between your on-premises Exchange Server and Office 365, you need to set up a connector. Set up connectors to route mail between Microsoft 365 or Office 365 and your own email servers | Microsoft Learn
4. Considering that you have a hybrid environment, make sure that the hybrid configuration wizard HCW is running correctly and that there are no misconfigurations. Verify that the hybrid configuration settings are correct and that the necessary connectors are in place and functioning properly.

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