Sometimes you add permissions to mailbox folders for other users.
You can check permission to mailbox folder using Outlook but then you need to check each folder.
If you want to get list of permissions assigned for users to specified mailbox folders you can use below script.
In this script we use Get-MailboxFolderStatistics to get list of mailbox folders and subfolders, and Get-MailboxFolderPermission to get permissions assigned to those folders and subfolders.
When we connect both commands we will get script which will list all permissions assigned to all folders and subfolders in mailbox.
To check specified mailbox, assign mailbox alias or smtp address to variable $MBX_tocheck