Can't access a hidden mailbox in Outlook after a migration to Microsoft 365 hybrid environment

Original KB number:   4034273

Symptoms

After a migration to Microsoft 365, users can't open a folder or add a hidden mailbox in Outlook. Also, users may receive the following error messages, depending on what they're trying to add or access.

• Additional mailbox

  The name cannot be resolved. The name cannot be matched to a name in the address list.

• Additional Exchange account

  The name cannot be matched to a name in the address list.

• Calendar or other folders

  Microsoft Outlook does not recognizeUserName.

Cause

This behavior is expected when you try to add a hidden mailbox.

Resolution

Auto-mapping enables users who have Full Access permissions to access a hidden mailbox. If you migrate from an on-premises environment, refer to the following article for instructions to enable auto-mapping in a hybrid environment:

Auto-mapping doesn't work as expected in a Microsoft 365 hybrid environment