Hi 33DNClarke,
Thanks for posting in Apple Support Communities.
We understand that your Exchange mail account doesn't show subfolders. From Exchange, do you have Sync Settings enabled and allowing the account to sync IMAP Folders. You may need to contact Microsoft on specifics to enable these settings. If the correct settings are enabled from the mail account, remove the email account and add it back to your Mac.
1. In the Mail app
on your Mac, choose Mail > Settings, then click Accounts.
2. Select an account, then click the Remove button
.
Note: If the account is used by other apps on your Mac, you’re asked to remove the account in Internet Accounts settings. Click the button to open Internet Accounts, then turn Mail off for the account. To stop using the account in all apps, make sure the account is selected, then click the Remove button
.
These steps are found in Log out of or remove email accounts in Mail on Mac.
Regards.