So, my work email is an Exchange account (actually Office365) which I connect to via Mac Mail. I just noticed two new mailbox folders "Working Set" and "Working Set1" that appear in my mailbox list for that account. These folders don't show up in the Office365 web interface, nor do they show up on my iPad or iPhone. They also cannot be deleted. Has anyone else who has an exchange account on Mac Mail had these mailboxes appear recently?