Weird thing happen.
I have public folder_ABC and my outlook listed this public folder_ABC in the folder list. After I removed public folder_ABC using ECP but the public folder mailboxes for public folder_ABC still exist in ECP.
Issue happened when i tried to add reminder@contonso.com mailbox into my outlook, it listed as screenshot below.
I deleted the Public folder_ABC in ECP and it is listed as disabled in EMC now.
But my mailbox and data@contonso.com is still listed as public folder when click on folder list in outlook 2010...