Exchange Online users can't access legacy on-premises public folders
Original KB number: 3106618
The Hybrid Configuration wizard that's included in the Exchange Management Console in Microsoft Exchange Server 2010 is no longer supported. Therefore, you should no longer use the old Hybrid Configuration wizard. Instead, use the Microsoft 365 Hybrid Configuration wizard. For more information, see Microsoft 365 Hybrid Configuration wizard for Exchange 2010.
You have a hybrid deployment of Microsoft Exchange Online in Microsoft 365 and on-premises Microsoft Exchange Server, and you've configured on-premises public folders in Exchange Server 2010 or Exchange Server 2007. These folders are also known as legacy public folders.
In this scenario, a user who was created in Exchange Online receives the following error message when they try to access the legacy public folders:
Cannot expand the folder. The set of folders cannot be opened.
Network problems are preventing connection to Microsoft Exchange
This issue occurs if the mailbox for the user was created in Exchange Online and doesn't have an associated on-premises user object for authentication to the public folder.
For this issue to be resolved, the Exchange Online mailbox must be linked to an on-premises user account. For more information, see How to use SMTP matching to match on-premises user accounts to Microsoft 365 user accounts for directory synchronization.
For more information, see Configure legacy on-premises public folders for a hybrid deployment