As POP3 users have discovered, when you use rules to move email to another folder, the folders expand. As these POP3 users move to Exchange server, their folders don't expand.
“How can I get the subfolders to autoexpand when a new message arrives in one of these subfolders?”
This is not an option that can be turned off and on, the folders either expand when messages are moved into them or they don’t.
It works with POP3 accounts because Outlook applies the rules and moves your messages to folders as it downloads them. This expands the folder tree so that folders with new messages are visible. (Note that not all versions and builds support expanding folders but when they do, it should always work with POP3 accounts. )
When you use Exchange, messages may be moved by rules on the server, when Outlook is closed. When you open Outlook the messages are already in the folders and just need synced to Outlook.
How do you know if the rule will work when Outlook is closed? If the rule does not have “(client-side)” after the name, the rule runs on the server as the mail is dropped into the mailbox.
If a rule is marked (client-side) you can’t force it server-side (except by editing the rule to remove the client-side conditions), but server rules can be forced to the client-side by adding flags, categories, or other conditions that are applied by the client only. Such rules may expand the folders as mail is delivered. Note that messages waiting to be moved by client side rules will be in the Inbox when you view it using OWA or a smartphone.